[M2-TILE-01] Tilemap data + auto-depth from tile height (FR-2.6)

[offdev]

Implements M2-TILE-01 (roadmap/M2-rendering-2.5d.md): tilemap data + auto-depth from tile height (FR-2.6).

What

• laige::render::TileMap<Backend> (header-only, templated over the SimMath backends — the presentation.h pattern; TileMap::Options is the M2-ISO-02 table's Options, one type, no duplicated validation):
  • The chunked tile grid — per tile: textureId (the game-assigned atlas ref), height, animationId (data only in M2; M2-TILE-02 cycles frames from it).
  • Auto-depth wiring: the height lives in the owned IsoDepthKeyTable alone (one source of truth); setTile/rebuild route the heights into the table's incremental / from-scratch update — a tile's Y height is automatically reflected in its depth key (radius 0: a height edit changes exactly that cell's key).
  • declareTo(batcher, options) — the S-5 batch path: one SpriteItem per tile (tile-center pos (gx+0.5, gy+0.5), scale (1,1), rotation 0, the fixed-frame UV, the table's key — depthOverride stays false, G-R11, the tile's texture as atlasId) in the grid's row-major order (RENDER-003 — the tile's grid position is its stable identity). The batcher's (atlas, material, blend) grouping renders a tilemap in a bounded number of draw calls: one per distinct (textureId, material, blend) combination — tiles of one chunk sharing one texture and blend form one group.
• Rejected operations leave the tile data and the table unchanged (validated before any write; the Status is the failure channel — LOG-002); the happy update/declare paths log nothing.
• ARCH-009: headless-buildable, presentation-only, sim-phase writes / render-phase reads; the declare loop allocates nothing per frame (FR-2.2 — pinned by a 1000-frame zero-allocation test).

Tests (CTest entry tilemap — the step's Verify command)

17 tests / 6 suites, no GL (runs in every tree): grid options + flat/empty contract, per-tile writes / rejected edits / scene load, the rebuild-from-scratch == incremental property, the height change → depth-table increment (roadmap unit test, radius-0 pin + hand-computed golden + independent oracle), the hand-computed 4×4 chunk's tile quad positions/depth goldens (16 literal keys, the exact tie structure) + grouping (2 textures → exactly 2 groups, ascending order, hand-computed in-group instance sequences) + cross-frame determinism, the frame protocol / failure paths / custom options / no-log happy path, and the zero-allocation declare loop.

Docs (same change)

NEW docs/api/tilemap.md; docs/README.md index; the module README; docs/concepts/coordinates.md (new §4.9 + the §5 conversion row + Related); roadmap box + board (M2 17/33, total 63/194) + change-log row.

API surface / budgets / compat

• laige-api.json regenerated: 1233 → 1265 symbols, 38 → 39 headers (+32 additive).
• No standalone budgets.json entry (count stays 16 — the BudgetHarnessTable pin): the per-frame declare cost is part of the composite 50k render-CPU budget (sprites_50k_cpu, M2-PERF-01).
• Additive only — no existing symbol's signature or meaning changed.

Verification (all local, no GPU)

• All six trees warning-clean + full ctest green: build 112/112, build-clang 112/112, build-release 101/101, build-shared 112/112, build-asan 109/109, build-tsan 109/109.
• ctest -R tilemap green (17/17); api-real-tree/api-check-fresh (6/6), tools/laige-include-lint (68 source files), tools/laige-determinism-lint (28 sim files, 0 violations) green.

Note for the CI lanes: MSVC is not compiled locally — the new code path uses only explicit casts (no implicit size_t→u32/int narrowing); the ci:windows label gate applies as usual.