wip(assembler): M1 round 1 -- manifest format, block contract, bound audit, native Heightfield2D

Luna, M1 round 1 of 4 (plan in Standalone/ASSEMBLER_M1_REPORT.md):
- Versioned fixed-scale world manifest (the shipped plan format) and the Core block
  contract: point evaluation, interval-or-unknown, influence bound, detail envelope, channels.
- Interval audit: 256 boxes / 6,912 samples, 0 missed, 0 unknown; a deliberately false bound
  is detected (negative control).
- Native Heightfield2D with biome input: 4,096-point hash fae2721c052aec5c identical at
  W1/W8/W16; point queries 2.059 ms vs 62.115 ms legacy -- but NOT a match yet (mean density
  error 274 voxels, 2,261 sign mismatches vs the legacy surface). 45 mesh-free Core queries
  match direct evaluation exactly. Existing cave world unchanged (walkability, 64/64 tiles).
- Open: collision readiness, a valid full-view timing (viewer captures were occluded/empty;
  top-down height maps are the only usable pair).
Also carries the design doc's owner decisions (fixed water; player edits unrestricted, route
law for automatic changes only) and three plugin files + BuildHost scripts that earlier round
commits had missed (also fixed on unreal/core-integration as 2574b56).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/DESIGN_WORLD_ASSEMBLER.md

@@ -770,3 +770,18 @@ and shape it, then you have my go on another branch so nothing is ever ruined an
 starts on branch `assembler/world`, created from this commit.
 
 -- Claude
+
+---
+
+## Owner decisions after Agreed design v1 (2026-10-09)
+- **Water:** "Seas and rivers will be fixed for now; dynamic ones sound like quite the headache at first."
+  Fixed-season water bodies are the product decision.
+- **Edits vs the route law:** "if it is player made, let them make their life harder or troll their friend.
+  If it's automatic, then yeah we don't allow it." So:
+  - **Player edits (carve and fill) are unrestricted.** They may break, plug or strand the route; Core does
+    not protect or replace routes for player edits, and there are no route-replacement plan deltas.
+  - **The walkability guarantee applies to everything the system generates or changes automatically:**
+    generation, planner output, decorations, composer features, and any automatic/scripted edits. Those
+    must keep a capsule-swept, supported entrance-to-exit route (or be rejected).
+  - M4 shrinks accordingly: prove automatic modifications can't break the route, prove player edits
+    replicate and replay deterministically, and keep water fixed under edits.