Reduce congestion in dense layouts

[konojunya]

Background

Dense diagrams could route unrelated edges through the same node-side midpoint or a nearly identical corridor. That produced ambiguous junctions and visually crowded local regions even while the existing collision gates remained green. Built-in cache nodes also still used the decorative cylinder silhouette removed from database nodes.

Implementation Approach

• Add deterministic offset terminal slots only where a node side has multi-edge demand.
• Penalize reused terminals, shared segments, and close parallel lanes while preserving bounded deterministic routing.
• Align same-rank nodes with connected external neighbors unless authored order is explicit.
• Keep independent edges separate instead of inventing implicit shared trunks.
• Consume published stack-theme 0.7.0 and advance native/WASM Engine packages to 0.9.0.
• Record the routing decision, release impact, and visually approved scene/SVG snapshots.

Testing Approach

• 150 native unit tests and 157 conformance-feature tests.
• Seven-case approved layout corpus plus text, edge, and group-frame hard geometry gates.
• Congestion fixture requiring zero reused terminals, proper crossings, ambiguous junctions, shared route length, and close parallel length.
• 14 browser WASM tests with native parity and 60 labelled-DAG regressions.
• Rust stable and Rust 1.85 tests, Clippy, Rustdoc, SVG validation, package checks, Cargo publish dry-run, and release-mode budgets.
• Browser visual review of the supplied Sample Service and all node kinds.

Review Perspectives / Points of Concern

• Confirm offset ports remain opt-in for congested sides and do not destabilize simple diagrams.
• Confirm explicit authored order still overrides neighbor alignment.
• Confirm independent edge identity and arrow semantics remain visible without shared trunks.
• Confirm approved snapshots reflect intentional geometry changes and the cache card from Theme 0.7.0.