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/Core/API.md

@@ -79,6 +79,45 @@ Pass `analyticOnly = false` to retain the reference packed-page compiler. Packed
 parent-cell coordinates and an 8-voxel apron; each covers a 128-voxel cube or one larger coarse
 tile. This path remains available for parity checks and workloads that need the packed lattice.
 
+## World assembler manifest v1 (prototype)
+
+`ResolvedWorldSnapshot::FromJson` accepts an optional `world_assembly` object alongside the owner
+snapshot. The object is the shipped-plan payload shape: a fixed-scale recipe and finite-strate plan
+with hashes, no generated mesh or voxel samples. The reviewed sample payload is
+[`round1-manifest.json`](../Research/WorldAssemblerM1/round1-manifest.json). Round 4 still needs to
+connect this payload to the season asset/hash path.
+
+Every coordinate and scalar uses signed integer units of 1/256 voxel (`units_per_voxel: 256`),
+independent of the plugin's 25 cm Core voxel and of tile size. Schema and evaluator versions are
+both 1. JSON integer fields are canonicalized in declared field order and hashed as 4- or 8-byte
+little-endian words with FNV-1a 64. Hashes are emitted as 16 lowercase hexadecimal characters;
+JSON key order and whitespace do not affect them. The root, recipe, plan and each stratum carry a
+content hash. The recipe stores its seed, warped biome-map settings and one to eight biome profiles;
+the plan stores finite-strate extents, cap settings, stable-ID features and route points. Bounds,
+feature count and route count are validated before a manifest is accepted.
+
+```cpp
+std::string error;
+auto manifest = VoxelForgeCore::WorldAssemblyManifest::FromJson(payload, error);
+if (!manifest || !manifest->validate(error)) { /* reject the payload */ }
+const auto* loaded = snapshot.worldAssembly(); // null when the owner snapshot has no plan
+```
+
+`Heightfield2D` is the first field block. `point(xQ, yQ, zQ)` evaluates the biome map and blended
+height profiles at fixed-scale coordinates, returning the ground and cap heights, primary and
+neighbor biome IDs, Q16 blend weight, and signed density. Negative density is solid. Its block
+contract reads `biome` and writes `height` plus `rock`; `water`, `waterSurface`, `route` and
+`surfaceTags` are declared channels for later blocks. The detail envelope reports maximum absolute
+vertical displacement and minimum feature size. Influence is global in XY and limited to the
+finite stratum's Z extent.
+
+`interval(minXQ, minYQ, minZQ, maxXQ, maxYQ, maxZQ)` returns a conservative density range or
+`known == false`; callers must keep evaluating when it is unknown. The current heightfield range
+uses the maximum recipe-profile amplitude and cap undulation over the whole biome set. The public
+`AuditHeightfieldBounds` samples points inside randomized boxes and includes a deliberately false
+one-sided range as a negative control. A pass means no sampled point escaped and the negative
+control was rejected; it is an audit of this block implementation, not a proof for future blocks.
+
 ## Queries and tiles
 
 `densityAt(position)` returns signed density; negative is solid and nonnegative is air.