Keep arrowheads clear of terminal bends

[konojunya]

Summary

• extend the final orthogonal run to 16px for arrow-marked endpoints by shifting the adjacent two-bend section after grid routing
• keep arrowless endpoints and the routing search grid unchanged so dense layouts retain their runtime characteristics
• validate each adjusted path against canvas bounds, nodes, fixed text obstacles, and frames, retaining the original route when a local shift is unsafe
• add forward, bidirectional, and blocked-shift regressions and update reviewed scene and SVG snapshots

Verification

• cargo test --workspace --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo +1.85.0 test --workspace --locked
• cargo +1.85.0 clippy --workspace --all-targets --all-features --locked -- -D warnings
• canonical formatter and engine conformance suites
• coverage thresholds: lines, functions, and regions all above 95%
• layout text, edge, frame, congestion, and release runtime gates
• python3 scripts/validate-svg.py
• native/WebAssembly parity, TypeScript, and npm package validation
• local before/after visual review of the affected Order database and Receipt archive arrows