Lane D R18 (v4.0-alpha): citizens on real bones — audit + fragmentation finding (ledger #5)

Audited the citizen sim on real irregular street topology. E landed katoomba_real's
real roads (484 ways -> 872 nodes/799 edges) mid-round, so this is measured on the
real Katoomba graph plus a crafted irregular proxy.

VERDICT: the CityPlan graph contract IS the contract — zero sim changes. On real
Katoomba (7.2deg junctions, 1303m edge, 105 components): 0 NaN over 1,095,500 samples,
peds footpath-perfect (0 off-footpath), loiter/bench-sit/patronage all fire, chunk
streaming works over non-grid, determinism byte-identical (1167 identity sigs, R14
chunk-local law holds). No source touched.

FILED FOR A: katoomba_real's graph is 105 components — main holds 58% of street-metres
+ 18/20 shops, but 42% of street-metres + 2 shops strand (~10% of near-main crowd
presence lands on tiny fragments). Primary cause is expected: median fragment 71.5m
from main = peripheral/bbox-clipped roads. Secondary (corroborated with Lane B on A's
selfcheck fixture): per-way Douglas-Peucker drops collinear junction points + no
mid-edge split — bites only the 9/85 fragments <10m from main. Reconciles with B:
B verified 126 intersections form (main component); I add the global measurement
(still 105 components). A fidelity knob (charter risk #4), not a blocker; sim degrades
gracefully (no crash). A owns the connect-tolerance decision.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/LANES/LANE_D_NOTES.md

@@ -6,6 +6,72 @@ _rotOnly/head-bone normalize/upgradeStreetPeople). Measurements on the M3 Ultra
 
 ---
 
+## ROUND 18 — v4.0-alpha REAL ROADS: citizens on real bones (ledger #5)
+
+*PROCITY-D, 2026-07-16. The audit: does the citizen sim behave on real (irregular) street topology?
+E landed `katoomba_real`'s real road geometry (484 OSM ways → 872 nodes / 799 edges) mid-audit, so
+this is measured on the **real Katoomba graph**, not just a proxy. In-shell: fresh `CitizenSim` built
+on `plan.streets`, driven headless-of-render (rAF throttled). No source changes — the finding is the
+verdict + the failure list.*
+
+### Verdict: the graph contract IS the contract — the sim needs ZERO changes
+The sim's graph-walk is topology-agnostic by construction (seeded random walk over nodes/edges, dead-end
+U-turn), so real geometry should just work. It does. Proven on a **crafted irregular town** (deg-4
+crossroads, 3 T-junctions, 5 dead-ends, a 30° acute fork, a cycle) AND the **real Katoomba graph**
+(7.2° acute junction, a 1303 m edge, 105 components):
+
+| assertion | crafted irregular | **real Katoomba** |
+|---|---|---|
+| NaN/Inf ped positions | 0 / 240k | **0 / 1,095,500** |
+| off-footpath samples (ped beyond its own edge band) | 0 / 240k | **0** (maxDist 12.9 m = main offset) |
+| edges traversed | 11 / 11 | full main network |
+| peds pinned at dead-ends | 0 | 0 |
+| loiter fires | 154 ev | **174 ev** |
+| **bench-sit fires (R17)** | 47 ev | **60 ev** |
+| patronage fires (stream mode) | 165 visits | **24 visits, 4.3k inside-frames** |
+| chunk streaming over non-grid | 7 live rosters / 93 active | **13 chunks / 307 active** |
+| determinism (2 sims, same seed) | 0/120k pos mismatch; identity set visit-order-independent | **byte-identical rerun; 1167 sigs** |
+
+Identity signature format is intact (`chunkKey#i:pedIndex:pvar:height:speed:edge:forward`), no `sit`
+term — my R17 `benchsit` stream doesn't perturb it. **Nothing in the sim needed touching for real roads.**
+
+### The measured leak → FILED FOR A: the real-roads graph is fragmented
+`katoomba_real`'s street graph comes out in **105 disconnected components**. The main component holds
+**58% of street-metres (52.8 km) and 18 of 20 shops** — the core town is coherent — but **42% of street
+metres and 2 shops strand** on 102 islands (9 single nodes, 76 fragments of 2–4 nodes, 17 of 5–20).
+Live cost to citizens: **~10% of near-main crowd presence** lands on tiny fragments (aimless wandering,
+can't reach the main street or its shops); the 2 stranded shops get no main-crowd traffic.
+
+- **Primary cause — expected, not a bug:** on real Katoomba the median fragment is **71.5 m** from the
+  main net → **mostly peripheral / bbox-clipped roads and genuinely-separate service ways**, which is
+  what an OSM extract in a bounded box always contains. The main town is coherent; this is the tail.
+- **Secondary mechanism (corroborated with Lane B):** A's `buildRealRoads` runs Douglas–Peucker **per-way
+  before snapping**, so a junction point that is collinear on the through-way gets **dropped**, with no
+  mid-edge split where one way's endpoint lands on another way's segment — a near-straight through-street
+  with a mid-way junction can lose it → the crossing way strands. I hit this **three times** crafting the
+  test town (fixed by modelling junctions as shared way-endpoints, as OSM does); **Lane B independently
+  hit the same DP-collinear drop** on A's `selfcheck` fixture (LANE_B_NOTES §18). On *real* Katoomba this
+  bites only the **9 of 85** tiny fragments that sit <10 m from the main network (20 within 30 m) — the
+  ones a snap-tolerance / endpoint-split pass could legitimately join. A **fidelity knob** (charter risk #4).
+- **Reconciles with Lane B:** B verified real Katoomba's **126 intersections form correctly** (= my 27
+  deg-4 + 99 deg-3 junction nodes) and concluded connectivity is fine — true *for the main component*. My
+  add is the **global** measurement B didn't take: those 126 junctions live in a 480-node main component,
+  but the full graph is still 105 components with 42 % of street-metres on islands. Both true, compatible.
+- **The sim degrades gracefully** — fragment peds just walk their island; **zero NaN, no crash**. This is
+  A's graph-construction call (connect-tolerance / mid-edge split), not a sim change. A D-side mitigation
+  (bias spawns toward the main component) is a **beta** item and needs A's connectivity decision first.
+
+### Handshakes
+- **D → A:** the fragmentation measurement above (105 components, 42% street-metres + 2 shops stranded,
+  root cause + the 9 close-fragments that could be joined). Your knob, your call.
+- **D → F (gate):** the sim is clean on the real graph — no D-side gate concern. Bench-sit + loiter +
+  patronage + determinism all hold on `katoomba_real`. When you re-run the town matrix with the roads
+  dimension, the citizen path is green from my side.
+- **Re-run note:** the whole audit is parameterized on `plan.streets`; it re-runs on any town's real
+  graph in one call (used it on both the crafted town and real Katoomba this round).
+
+---
+
 ## ROUND 17 — v3.2 THE TAIL: the drummer's pelvic-lean (ledger #1)
 
 *PROCITY-D, 2026-07-16. The R16 honest limit — the upright bent-leg sit read stiff — is fixed. Files