Add deterministic scene layout

[konojunya]

背景

Task 12 requires a renderer-independent scene model whose node and nested-group geometry is deterministic across native and WebAssembly targets. The engine previously stopped at the facade boundary and did not validate layout feasibility.

実装方法

• Add fixed-width integer scene geometry, catalog-only font measurement, recursive node/group sizing, and explicit padded content rectangles.
• Implement right, down, same-rank, cross-axis order, and a documented per-scope automatic direction policy.
• Validate containment, positive bounds, identifier uniqueness, and sibling non-overlap from check and valid render attempts.
• Add an ADR, canonical complete-semantics scene snapshot, browser-target build, and a shared numeric fixture executed in both native and WASI test binaries.

テスト方法

• cargo +stable test --workspace --locked
• canonical formatter and scene conformance tests against the pinned specification revision
• native and wasm32-wasip1 numeric parity test with Wasmtime 48.0.1
• wasm32-unknown-unknown engine build
• stable and Rust 1.85 Clippy with warnings denied
• rustdoc with warnings denied
• llvm-cov: 97.00% lines, 97.66% functions, 95.86% regions

Review 視点 / 懸念点

• Automatic direction intentionally resolves to right for up to three direct children and down for four or more, independently in every nested scope.
• Geometry uses milli-pixel i64 values and no platform font APIs. Adapter-level full-output parity remains scoped to the later WASM adapter task.
• The scene remains internal so edge routing and SVG serialization can extend it without exposing a premature public API.