Add hierarchy-aware bounds API

[MatthewMckee4]

Summary

Add an authoritative hierarchy-aware bounds report with cached child bounds, transformed instance bounds, deterministic diagnostics, and explicit empty/cyclic results. The viewer now consumes the core bounds logic and all deep hierarchy paths use heap-backed traversal. Closes #316.

Test Plan

Full workspace tests, strict Clippy, warning-denied rustdoc, docs build, and pre-commit hooks.

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks
🆕 2 new benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Simulation	deep_chain_10k	N/A	111.1 ms	N/A
🆕	Simulation	shared_child_50k_arefs	N/A	139.6 ms	N/A

---

_{Comparing codex/hierarchy-bounds (a38c1b5) with main (923818c)}

[codecov]

Codecov Report

❌ Patch coverage is 94.19866% with 139 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/gdsr/src/hierarchy_bounds.rs	95.57%	58 Missing ⚠️
crates/gdsr-viewer/src/bevy_scene.rs	89.63%	46 Missing ⚠️
crates/gdsr-viewer/src/drawable.rs	92.94%	29 Missing ⚠️
crates/gdsr/src/elements/reference.rs	96.49%	6 Missing ⚠️

Files with missing lines	Coverage Δ
crates/gdsr/src/elements/gds_box.rs	96.95% <100.00%> (+0.48%)	⬆️
crates/gdsr/src/elements/path.rs	98.37% <100.00%> (+0.01%)	⬆️
crates/gdsr/src/geometry/bounding_box.rs	99.03% <100.00%> (+0.25%)	⬆️
crates/gdsr/src/elements/reference.rs	99.46% <96.49%> (+0.73%)	⬆️
crates/gdsr-viewer/src/drawable.rs	79.01% <92.94%> (+15.69%)	⬆️
crates/gdsr-viewer/src/bevy_scene.rs	87.20% <89.63%> (+3.87%)	⬆️
crates/gdsr/src/hierarchy_bounds.rs	95.57% <95.57%> (ø)

... and 1 file with indirect coverage changes

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