Add deterministic standalone SVG rendering

[konojunya]

背景

Task 14 requires the pure engine pipeline to produce safe, deterministic standalone SVG after compilation, theme resolution, layout, and routing.

実装方法

• Resolve requested themes and icons with source-mapped STK6001 and STK5001 fallbacks.
• Serialize groups, all node shapes, routed edge kinds and markers, labels, details, embedded catalog icons, provenance, and accessible metadata without host I/O.
• Escape authored text, restrict references to the local arrow marker, and place edge labels deterministically outside node bounds.
• Add canonical SVG snapshots, XML/security validation, render regression tests, and ADR-0005.

テスト方法

• cargo +stable fmt --check
• cargo +stable test --workspace --locked
• canonical formatter and engine conformance suites
• python3 scripts/validate-svg.py
• wasm32-unknown-unknown build
• wasm32-wasip1 numeric parity with Wasmtime 48.0.1
• cargo +stable clippy --workspace --all-targets --all-features --locked -- -D warnings
• warnings-as-errors rustdoc
• Rust 1.85 workspace tests and Clippy
• cargo llvm-cov: regions 95.50%, functions 96.95%, lines 97.07%
• Chromium visual review of default and complete canonical SVG at fixed viewports, with zero page or console errors

Review 視点 / 懸念点

• Confirm warning-only missing resources preserve a fallback SVG and authored diagnostic ranges.
• Review the standalone SVG security boundary and embedded catalog-icon assumption.
• Review deterministic edge-label collision avoidance and exact snapshot changes.