Fable: THE FRONTIER gains THE THIRD DIMENSION + TEXTURE LIFE — the noise ruling (semantic streams stay; noise only where continuous), terrain chartered v8+ with costs named

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/THE_FRONTIER.md

@@ -59,6 +59,37 @@ makes digging MEAN something is unbuilt:
 - **The 90s-ness:** it's in the fabric (video shops, milk bars) but no system USES the era —
   no phone-free navigation puzzles, no street-press gig guide, no mixtapes.
 
+## THE THIRD DIMENSION + TEXTURE LIFE (filed 2026-07-18 — John's Perlin question, answered from the code)
+
+**The ruling that frames this section:** world *structure* stays on the semantic hash-stream
+system (`prng.js` — `seedFor(citySeed, kind, id)`, independently-keyed streams per entity).
+That architecture is why 31 rounds of features never moved a golden: a draw added to one
+stream shifts no other. Perlin answers a different question — a *continuous* value over
+space/time — and it enters the toolbox only where the question really is continuous. When it
+does, it seeds from the same citySeed keying: deterministic, gate-able, no exceptions.
+
+Candidates, ranked by honest cost:
+
+1. **TERRAIN / ELEVATION — the headline, epoch-scale, chartered as a v8+ candidate.**
+   Everything is flat, which is *wrong for the real map*: Katoomba is a mountain town rendered
+   as a billiard table. Per the real-map DNA, the primary source is REAL elevation (SRTM /
+   contour data for each town bbox, the same discipline as the roads fetch) — noise is only
+   the infill between samples, never the shape of a real hill. **The cost is named now so
+   nobody charters it lightly:** terrain touches everything — roads follow grades, buildings
+   need foundations and steps, footpaths slope, the chunk streamer and the walkers and every
+   budget meet a Y axis, and the classic covenant needs a flat-forever fence. This is an
+   epoch with an alpha/beta/close arc of its own, not a round.
+2. **Sky / cloud domes — cheap, near-term (v6/v7 polish).** Skies are baked textures today; a
+   seeded FBM dome gives every day its own weather-shaped sky, composes with the existing
+   weather states, and is one lane's item (B, lighting).
+3. **Wear & grime — cheap, high-charm (E/C canvas painters).** Seeded value-noise for sleeve
+   ring-wear, facade sun-fade, footpath staining — per-item, deterministic, and exactly the
+   90s patina the parody sleeves deserve. Pairs with the beta's price-guide work (a worn
+   sleeve *reading* worn next to its condition grade).
+4. **Crowd ebb (LIVING STREET track, D).** A slow 1D noise curve modulating street density
+   across the day — mornings trickle, arvo swells — richer than uniform draws, one stream,
+   identity-safe.
+
 ## The honest sequencing opinion (Fable's, not a ruling)
 
 The private-test deploy is the design instrument: John walks the game cold and writes down