[M2-SCENE-01] Reference isometric scene generator (deterministic, seed-driven)

[offdev]

M2-SCENE-01: Reference isometric scene generator

Implements roadmap step M2-SCENE-01 (roadmap/M2-rendering-2.5d.md, marked shipped): a headless, seed-deterministic generator for the worst-case isometric reference scene of PRD §4 — the scene every visual and performance acceptance test renders (downstream: M2-PERF-01, M2-AC-01, M2-GOLD-01, M2-SAMPLE-01).

What lands

• New public header src/laige-render/include/laige/render/iso_reference_scene.h (header-only, no GL calls):
  • buildIsoReferenceScene(seed, variant) — the Full variant is the §8.1 budget scene: 50 000 sprites on a 256×256 stepped-terrain grid of uniform-height plateaus (each sprite standing on its PRNG-chosen tile; positions exact dyadic 2^-16 — no FMA-susceptible composition, ARCH-010), 3 preset-factor parallax layers, 8 continuous particle emitters, a 2-widget UI on the 1280×720 reference screen. The Small variant scales the same composition to 1 000 sprites / 32×32 for fast unit tests. Same seed → same bytes, every build and P0 platform.
  • A versioned little-endian binary scene format (v1): the reader treats the bytes as untrusted input (SCALE-004) — full domain validation of every field plus the exact total length; any violation fails the whole read with MalformedInput; the version gate rejects anything but 1 (ARCH-007).
  • writeIsoReferenceScene — atomic temp + rename (a killed run never leaves a partial file).
  • IsoReferenceSceneWorld<Backend> (move-only) + loadIsoReferenceScene (first failure wins) + declareIsoReferenceScene — the one-call whole-world pass (parallax + terrain tiles + 50k sprites with isoDepthKey depth keys + live particles), O(frame items), zero allocation. The scene's atlas plan pins 14 (atlas, material, blend) groups — under the §8.1 30-draw-call budget, asserted by the tests.
• Headless tool tools/scene-iso/laige-scene-iso (--seed/--variant/--out): atomic write, round-trip self-check (byte-identical + clean re-parse), machine-greppable summary line, exit 0/2.
• Tests (tests/laige-render/scene_iso_tests.cpp, ctest -R scene_iso): builder determinism (same-seed byte identity both variants, different-seed differ), the 7 malformed-input cases, round-trip deep equality, load + declare on both backends (per-batch instance counts, in-group depth-key monotonicity, the 14-group draw-call property), the UI pass (2 quads / 1 batch / 2 depth overrides), and the 1 000-frame zero-allocation declare loop (both backends).
• Determinism gate scene_iso_cli (ctest): the tool twice per variant with the same seed, output files compared byte for byte — the roadmap Verify clause in executable form.
• Fuzz fuzz_scene_iso_parse (1 000 bounded runs every commit, corpus = valid small + full scenes as mutate/truncate bases) for the untrusted-input parser surface (TEST-005 / SCALE-005).
• Docs: docs/api/iso_reference_scene.md (API table, wire layout, Performance section, misuse warnings), docs/README.md index link, roadmap Shipped block. laige-api.json regenerated last (1815 symbols, 48 headers).

Engine bug fixed along the way

The scene loader's first use of the StringTable move hit a real bug: StringTable::operator=(StringTable&&) recursed into itself (*this = StringTable(std::move(o)) is again a move-assignment → infinite recursion, stack overflow). Fixed in src/laige-render/include/laige/render/text_items.h (member moves applied in place); regression test TextStringTable.MoveAssignment.

Notes

• budgets.json unchanged (17 entries): the generation budget is the tool's (the scene_iso_cli 60 s timeout guards it), and the per-frame declare cost is part of the composite 50k render-CPU budget measured by M2-PERF-01.
• GL-creating ctest entries were not run locally (GPU policy); they are CI-covered, as with prior steps.
• Local Linux gate (all 4 trees, full ctest minus the GL entries): gate-gcc 115/115, gate-clang 115/115, gate-asan 112/112, gate-tsan 112/112; include-lint clean.