Add deterministic edge routing

[konojunya]

背景

Task 13 requires the internal scene to preserve edge semantics, route connectors outside node interiors, place labels deterministically, and report unsatisfied order hints at their authored source ranges.

実装方法

• Add ordered scene edges with normalized kind, direction, label, orthogonal path, endpoint markers, and integer half-path label anchors.
• Build one deterministic rectilinear visibility grid from canvas margins and node geometry, then route each edge with Manhattan distance, a fixed bend penalty, and stable tie ordering.
• Validate boundary endpoints, axis-aligned nonzero segments, canvas containment, marker semantics, label anchors, and absence of node-interior intersections.
• Evaluate each order hint on the resolved cross-axis and append STK4001 only when the placed centers do not satisfy it, using the compiler source-map statement span.
• Expand the canonical scene snapshot with all eight complete-semantics routes and run every public specification example through routed-scene validation.

テスト方法

• cargo +stable test --workspace --locked
• canonical formatter suite and full scene conformance suite against the pinned specification revision
• native and wasm32-wasip1 geometry plus routing numeric fixtures 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: 96.97% lines, 97.94% functions, 95.69% regions

Review 視点 / 懸念点

• Forward edges use a target arrow, bidirectional edges use both endpoint arrows, and associations use no arrow.
• Paths may touch or follow node boundaries but cannot enter a node interior.
• The bend penalty improves readability but does not promise globally minimal crossings; that remains a renderer quality goal rather than source semantics.
• Compiler diagnostics remain first, followed by layout warnings in diagram then depth-first group order.