[M2-SCENE-01] Reference isometric scene generator (deterministic, seed-driven) - #93

…d-driven)

The M2-SCENE-01 step: a headless, seed-deterministic generator for the
worst-case isometric reference scene of PRD §4 — the scene every
visual and performance acceptance test renders (M2-PERF-01,
M2-AC-01, M2-GOLD-01, M2-SAMPLE-01).

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 256x256 stepped-terrain grid of
  uniform-height plateaus — each sprite standing on its PRNG-chosen
  tile, positions exact dyadic 2^-16), plus the 3 preset-factor
  parallax layers, 8 continuous particle emitters, and a 2-widget UI
  on the 1280x720 reference screen; the Small variant is the same
  composition scaled to 1 000 sprites / 32x32 for fast unit tests.
  Same seed, same bytes, every build and P0 platform (ARCH-010 —
  Prng substreams + the exact dyadic position scale, no
  FMA-susceptible float composition in the builder).
- The 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 (ARCH-007 — the
  version gate rejects anything but 1).
- writeIsoReferenceScene: atomic temp + rename (a killed run never
  leaves a partial file).
- IsoReferenceSceneWorld<Backend> (move-only) + loadIsoReferenceScene
  (first failure wins; TileMap in unique_ptr — the house
  empty<=>stopped idiom, TileMap has no default constructor) +
  declareIsoReferenceScene: the one-call whole-world pass (parallax
  + terrain tiles + 50k sprites with their 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 test suite.

New headless tool tools/scene-iso/laige-scene-iso (the step's
generator CLI): --seed/--variant/--out, atomic write, round-trip
self-check (byte-identical + clean re-parse), machine-greppable
summary line, exit 0/2.

Verification:
- tests/laige-render/scene_iso_tests.cpp (ctest -R scene_iso): builder
  determinism (same-seed bytes identical, different-seed differ,
  both variants), 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).
- The scene_iso_cli ctest entry runs the roadmap's Verify clause in
  executable form: the tool twice per variant with the same seed,
  output files compared byte for byte.
- The scene format's untrusted-input surface gets a fuzz target
  (fuzz_scene_iso_parse, 1 000 bounded runs every commit, corpus =
  valid small + full scenes as mutate/truncate bases — TEST-005 /
  SCALE-005).

Also fixed (found by the scene loader's first use of the move):
StringTable::operator=(StringTable&&) recursed into itself
(`*this = StringTable(std::move(o))` is again a move-assignment —
infinite recursion). The move assignment now moves the members in
place; regression test TextStringTable.MoveAssignment.

docs/api/iso_reference_scene.md (the API + format + Performance
section), the docs/README.md index link, the roadmap Shipped block,
and laige-api.json regenerated last. budgets.json unchanged (17
entries — the generation budget is the tool's; the per-frame declare
cost is part of the composite 50k render-CPU budget, M2-PERF-01).

Unary minus applied to kIsoDepthLayerBias (a uint32 constant) before
the int32 cast warned C4146 on MSVC (treated as an error by the
house policy). Cast to int32 first, then negate — the iso_depth_key.h
clamp's own pattern (-static_cast to the signed type).