Add representative layout regression corpus

[konojunya]

Summary

• add a schema-versioned seven-case corpus covering small, medium, and dense diagrams, nested groups, rank and order constraints, cross edges, multilingual labels, and caller-owned provider icons
• compare current Engine SVG output byte-for-byte with approved references while validating geometry inventory, accessibility metadata, safe local-only content, and exact fixture inventory
• generate a responsive static approved/current review gallery with release-mode p95 and suite budgets, and enforce the contract in CI

Validation

• cargo test --workspace --locked
• canonical formatter and Engine conformance suites against the pinned specification revision
• cargo +1.85.0 test --workspace --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo doc --workspace --no-deps --locked with warnings denied
• cargo llvm-cov --lib --workspace --all-features --locked --fail-under-lines 95 --fail-under-functions 95 --fail-under-regions 95
• native and wasm32-wasip1 exact geometry parity with Wasmtime 48.0.1
• npm run build:wasm, npm test, npm run typecheck, and npm run pack:check
• release corpus benchmark: 91.517 ms total; slowest case 2.152 ms p95 under the 50 ms per-case and 2,500 ms suite budgets
• cargo audit and npm audit: no known vulnerabilities
• isolated Chrome review at 320 px and desktop: no console errors or external requests; Lighthouse mobile accessibility, best practices, SEO, and agentic browsing scores all 100; LCP 111 ms and CLS 0.00