# LANE C — venue interface contract (v3.0-FROZEN: the district) → Lane D + Lane F

> **v3.0-FROZEN** · 2026-07-16 · durable contract for Lane D + Lane F. Describes the *shipped* state;
> changes require a version bump (CITY_SPEC amendment law). Verified R13 (build) + R14 (release audit).
> **v3.1 amendment (2026-07-16):** §3 becomes the `stage.backline[]` contract (ledger #5 — the amp
> unification); C dropped its primitive ampStacks. Everything else in this doc is unchanged and stands.

*The district ships **three** venue archetypes behind `?gigs=1` — `pub`, `band_room`, `rsl`. Lane A
converts a chosen shop to a venue kind in place (`shop.type = kind`); C keys the interior recipe off
`shop.type`. All three return the SAME shape (below), so Lane D drops rigs in and Lane F reads state with
no per-kind branching. Non-venue rooms are byte-identical (venue code runs only for `recipe.venue`, on its
own seed sub-streams). Verified fresh (seeds 20261990/1990/7/42/101 + 90-build soak): 0 throws · 0 carves ·
0 path-fails · 0 determinism-fails · leak geo0/tex0 · worst build 8.3 ms. Budgets in §3.*

---

## 0. The pose `ry` convention (pinned) — READ THIS FIRST

**Canonical convention (every C pose uses it):** a pose's `ry` is the yaw of a thing whose **front /
outward normal is local −Z at ry=0**, i.e. world-facing `(−sin ry, −cos ry)`. `counter.stand`,
`browsePoints`, `watchPoints` and `bandPoses` ALL follow it, and the box placeholder (`placeholder.js`) is
built to it (toes at −Z). `keepers.js` greet math assumes it. **Drop a rig in at a pose's `ry` and it faces
the right way — no per-consumer flip.**

**Why no flip (the R13 fix, for the record):** the GLB fleet meshes ship facing local **+Z** at ry=0 — the
opposite of the −Z convention (verified: the anterior toe-bone vector `toe − foot` reads `(≈0, +0.99)`).
Before R13 this was compensated per-consumer, inconsistently: `band.js` flipped by π (band/crowd correct,
placeholders backward) while `keepers.js` did not (placeholders correct, **keepers + browsers 180° backward
whenever the GLB fleet was loaded** — they'd been shipping back-to-front). **D fixed it at the source in
`rigs.js buildFigure`: the cloned `inner` is rotated π about Y so every GLB rig faces −Z**, matching the
placeholder and every C pose. All consumer flips were then deleted. Net: keepers, browsers, band and crowd
all face correctly, GLB and placeholder modes are consistent, and C's pure-data poses never moved (no golden
touched). If you add a new rig consumer, plant it at the pose `ry` with no flip.

---

## 1. What `buildInterior` returns for a venue (`shop.type` ∈ `pub` | `band_room` | `rsl`)

Everything below is **pure data on the return** (room-local coords, the ry convention in §0). Lane D drops
rigs straight in; Lane F reads state — no new lifecycle vs. the R9 keeper/browser seam.

### `room.stage` — the band stage, a **4-piece** (null for non-venues)
```js
room.stage = {
  x, z,            // deck centre (against the back wall)
  w, d,            // deck size (m) — varies by kind (table below)
  deckY,           // deck top surface height (m) — the front line stands here
  riserY,          // drum riser top height (m) = deckY + 0.16 — the drummer sits here
  frontZ,          // z of the deck lip (the crowd stands in front of this, +Z)
  bandPoses: [ { x, z, ry, role, y, seated? } ×4 ],
}
```
- `bandPoses[0..2]` = the **front line**: `role` ∈ `guitar` / `vocal` / `bass`, across the deck lip, `y = deckY`.
- `bandPoses[3]` = the **drummer**: `role: 'drums'`, `seated: true`, up-stage centre on the drum riser,
  `y = riserY` (= `deckY + 0.16`, the `fittings.js` stage riser box).
- **Lane D:** lift each member to its own `pose.y` (front line = `deckY`, drummer = `riserY`). `role` hints
  the instrument — E's GLBs via `opts.instrumentFor(role)` (electric_guitar/bass_guitar/mic_stand/drum_kit),
  primitives otherwise (asset law). `seated:true` → seated bob. `ry = π` already faces the audience (+Z) — no
  flip (§0).
- The band is a **4-piece** for v3.0 (front-line trio + drummer). Reverting to a trio would be a version bump
  (drop `bandPoses[3]`), not a runtime toggle.

### `room.watchPoints` — the audience, **cap by kind** (empty `[]` for non-venues)
```js
room.watchPoints = [ { x, z, ry, dance, slotIndex } ]   // seeded floor poses, all facing the stage centre
```
- **Crowd cap by kind: `pub` 8 · `band_room` 8 · `rsl` 12** (the RSL is the crowd-cap stress case). The room
  fills what geometry allows up to the cap; **the count IS the crowd cap** (F asserts crowd ≤ `watchPoints.length`).
- `dance` seeded **~⅓ true** ("a bit of both"). D: `dance:false` → stand-and-watch idle; `dance:true` → bob/sway
  with a **seeded phase offset** (use `slotIndex`/coords). Every point walkable, reachable, clear of the keeper
  stand + browse points + stage/PA. Deterministic per `shop.seed`.

### `room.counter` / keeper — unchanged
The bar is a normal counter; the barkeep is just a keeper at `room.counter.stand` (R9 seam). Faces the
customer correctly under §0.

### `room.audio.gigKey` — the live bed
`room.audio = { musicKey, toneKey, gigKey? }`. The manifest key **is** the gigKey, canonical form
**`gig-<genreKey>`** — no mapping table. F passes gig state via `opts.gig`:
```js
buildInterior(rslShop, THREE, { gig: { on: true } })   // → room.audio.gigKey === 'gig-covers'
```
- Genre resolves `opts.gig.genreKey` → the venue's own `shop.genreKey` (A sets it) → a kind default
  (`pub`→pubrock, `band_room`→grunge, `rsl`→covers). Present **only when a gig is on**.
- **Prefer `gigKey` over `musicKey`** while the gig plays. E's beds are `gig-pubrock` / `gig-grunge` /
  `gig-covers`; a missing key → silence (audio law), band still visible.
- C hand-builds `'gig-'+genreKey` (byte-identical to citygen's `gigKeyFor`, selfcheck-asserted) rather than
  importing it — deliberately, to keep the interiors lib standalone (it never imports citygen).

### The venue table (C-owned tuning)
| kind | archetype bias | ceiling | deck `w × d`, `deckY` | crowd cap | genre → gigKey | dressing |
|---|---|---|---|---|---|---|
| `pub` | wide / hall | 3.6 m | `0.58·W × 2.0`, 0.32 | 8 | pubrock → `gig-pubrock` | pub tables, sticky carpet, bar |
| `band_room` | cosy / pokey / wide | **3.4 m (low)** | `0.62·W × 1.6`, **0.20** (barely a riser) | 8 | grunge → `gig-grunge` | near-empty standing room, tin-shed dingy |
| `rsl` | hall / wide | **4.0 m (tall)** | `0.50·W × 2.4`, **0.40** | **12** | covers → `gig-covers` | flock walls, club carpet, bistro tables, members' bar |

## 2. Quiet-night variant
No `opts.gig` → a normal interior: `stage` + `watchPoints` are still returned as data, but there's **no
`gigKey`** (room-tone + maybe the seeded radio `musicKey`), and D spawns no crowd. A seeded dark night reads
true — the venue is just a quiet pub/club.

## 3. `stage.backline[]` — the amp slots (v3.1, ledger #5)

`room.stage.backline = [ { x, z, ry, y } ×2 ]` — seeded up-stage amp/spare-cab poses, **pure data** (C plants
no amp meshes as of v3.1). This is the unified backline: **Lane D plants E's `guitar_amp` GLB here** (its own
primitive fallback under `?noassets`) — one amp system, GLB when present. C's old primitive ampStacks are
**dropped** from all three venue recipes.
- **slot[0]** = stage-right (the primary amp) at `(stage.x + stage.w·0.36, deckY, stage.z − stage.d·0.22)` —
  identical to D's R15-verified plant, so wiring D's amp to `backline[0]` moves nothing on screen.
  **slot[1]** = stage-left spare cab (mirror). `ry = π` faces the audience (+Z, the §0 convention); `y = deckY`.
- **Guarantees** (verified fresh, seed 20261990, pub/band_room/rsl): every slot is **up-stage of the front
  line**, flanks the drum riser, **clear of all 4 bandPoses** (nearest-bass clearance **0.87 m pub / 0.70 m
  band_room / 0.92 m rsl**) and **all watchPoints** (≥1.95 m), and **on-deck** (within the deck footprint).
  Deterministic per `shop.seed`. **Lane D:** verify no interpenetration once wired (C's R15 clearance discipline).
- **Draw impact:** dropping C's primitives shaved ~4 draws off each empty venue shell (drawSweep glb-off:
  pub 122 · band_room 120 · rsl 131 ≤350); D's GLB/primitive at `backline[0]` adds it back on the gig night.
- *v3.2 (parked):* the drummer sit clip, instrument LOD, and C's `audioEmitter` idea.

## 4. Budgets (measured R14, seed 20261990, fresh context)
- **Empty shells** (`drawSweep` glb-off): band_room **124** · pub **126** · rsl **135** — town-wide worst
  161 (opshop/hall). ≤ 350. ✓ (identical to R13 — C geometry unchanged).
- **Gig-night interior draws**, D's `GigCrew` with the real instrument GLBs + amp (localdepot): pub **60** ·
  band_room **52** · rsl **71**. Asset-free (placeholder crowd, no GLBs) worst RSL **237** (16 rigs). ≤ 350. ✓
- **Tris**: fleet peds ~2.7k each; E's instrument GLBs **14k each** (decimated to the ≤15k target), 5 on a
  stage = 70k — the instruments dominate. RSL gig-night ≈ **182k tris** once the async GLBs settle. The
  interior gate is **draws** (passes); tris are the *street* budget (200k), so this is not an interior gate.
  *Note for E/F:* F's gig-night smoke reports ~34k because it measures **before** the async instrument GLBs
  load; the settled scene is ~180k. A camera-distance LOD on instruments is a v3.1 candidate.
- **Determinism / release-law**: C's interior code is unchanged since R13; D's continuity + RY-fix ride
  disjoint rng namespaces (`ctx.stream(salt)` vs `rng(citySeed,kind,id)`) and cannot perturb C's placement
  or any golden. 90-build venue soak: 0 determinism · 0 carve · 0 path-fail · worst 8.3 ms; leak geo0/tex0.

## 5. The archetypes (C-owned)
`band_room` = a tin shed: small (cosy/pokey), low ceiling, barely-a-riser deck, near-empty standing floor,
bar in the corner — the PA louder than the room. `rsl` = carpet-and-flock club: the biggest floor in town
(hall/wide), a proper raised stage at one end, bistro tables round the edge, members' bar, the biggest crowd
cap. Both reuse the `pub` venue machinery (`theme.js venueSpec` + `layout.js` placeStage/placeWatchPoints);
only the dressing + `venueSpec` differ. Stage/PA keep GLB slots for Lane E (primitive fallback holds); the
primitive ampStacks are the backline fallback (see §3).