Add spatial grounding context extraction

[timfox]

Summary

• Add BuildEngineSpatialGroundingContext for clean-room extraction of USD-derived spatial/text cues from stages, including authored semantic label sets.
• Add usdSemantics::SemanticLabelsAPI for authored semantics:labels:<instance> token-array reads.
• Report semantic-label usage in engine snapshots and runtime support assessment (semantic_label_prim_paths, node label-set names, uses_semantic_labels).
• Merge origin/main (descendant world-bound aggregation, D bindings track, subdirectory embedding fix) and update spatial/engine tests in C++, C, Python, Go, and Rust to the new aggregation semantics.
• Link freeusd_usdSemantics in the D bindings so the C ABI resolves; enable Release asserts for the spatial grounding C++ test and fix a float-tolerance assert it surfaced.
• Mirror the validated surfaces through the C ABI, Python, Go, and Rust bindings; update engine contract, binding notes, module maps, and parity docs.

Testing

• cmake -S . -B build -DCMAKE_BUILD_TYPE=Release -DFREEUSD_BUILD_TESTS=ON -DFREEUSD_BUILD_PYTHON=ON
• cmake --build build --parallel
• ctest --test-dir build --output-on-failure (74/74 including pytest)
• go test ./... in bindings/go
• cargo test --manifest-path bindings/rust/Cargo.toml (35/35)
• dub test --compiler=ldc2 in bindings/d
• ./scripts/run_ci_locally.sh (all stages OK, including both D tracks)

Notes

• This does not embed MENTALMAP data, prompts, model calls, or benchmark claims; it only exposes fixture-backed scene cues and authored semantic labels that downstream systems can use under the existing GPL-compatible FreeUSD engine subset.