fix(layout): Harden layout internals, edge overlay keys, and WASM forceIterations

[mhiro2]

Summary

• Smooth out the auto-tuned density-factor curve and remove the saturating ceiling on excess_ratio so the compaction response stays continuous and keeps reacting past 2x the threshold.
• Speed up component-graph construction with HashSet-based dedup, replace silent unwrap_or_default calls in the SCC pass with descriptive expects, and precompute per-node radii so the spatial-grid repulsion no longer redoes spacing math per pair.
• Wire the obstacle-aware self-loop detour to the actual node extent so the endpoint anchor distance can be clamped on small nodes.
• Replace the collision-prone from->to string key in DiagramOverlay::edges with an explicit EdgeKey { from, to } tuple, and reject duplicate entries during deserialization.
• Plumb forceIterations through WasmRenderRequest / WasmDiffRequest with positive-and-bounded validation so JS callers can finally tune the force-directed layout safely.
• Add a property-based routing test and document the Mermaid v10 quoted-entity requirement.

Changes

• 70d7f72 : refactor(layout): tighten layout, rank, and routing internals
  • Make the density factor continuous at the 1.0 / 2.0 thresholds and stop saturating excess_ratio, so compaction stays responsive on large graphs.
  • Replace the O(C^2) component-adjacency contains check with a HashSet, promote SCC unwrap_or_default to descriptive expect so invariant violations surface immediately, and hoist node radii out of the per-pair repulsion hot loop.
  • Thread node sizes into the self-loop detour, add a property-based routing test (covering 96 random configurations), and document the Mermaid v10 quoted-entity requirement.
• e7c967b : fix(layout): key edge overlays by an explicit (from, to) tuple
  • Replace BTreeMap<String, EdgeOverlay> keyed by format!(\"{from}->{to}\") with BTreeMap<EdgeKey, EdgeOverlay> so identifiers that contain -> no longer collide.
  • Serialize the edge map as a list of {from, to, overlay} entries and reject duplicate entries on the way back in.
• 2a58355 : feat(wasm): respect forceIterations override from JS
  • Plumb the forceIterations request field through to_render_request / to_diff_request instead of pinning it to 150.
  • Validate the override against a positive lower bound and a 10000 upper bound so a runaway value can't freeze the host thread, and document the option on the public render/diff entry points.

[github-actions]

Code Metrics Report

	main (d1858b4)	#140 (dad45cd)	+/-
Coverage	94.9%	94.9%	+0.0%
Test Execution Time	1m26s	1m35s	+9s

Details

  |                     | main (d1858b4) | #140 (dad45cd) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          94.9% |          94.9% | +0.0% |
  |   Files             |             81 |             81 |     0 |
  |   Lines             |          39248 |          39477 |  +229 |
+ |   Covered           |          37253 |          37479 |  +226 |
- | Test Execution Time |          1m26s |          1m35s |   +9s |

Code coverage of files in pull request scope (94.6% → 94.8%)

Files	Coverage	+/-	Status
crates/relune-layout/src/diagram_export.rs	94.1%	0.0%	modified
crates/relune-layout/src/layout.rs	95.7%	-0.1%	modified
crates/relune-layout/src/overlay.rs	99.4%	+0.3%	modified
crates/relune-layout/src/rank.rs	98.4%	+0.0%	modified
crates/relune-layout/src/route/geometry.rs	95.6%	+1.6%	affected
crates/relune-layout/src/route/mod.rs	99.0%	+0.3%	modified
crates/relune-layout/src/route/obstacle.rs	93.6%	+0.1%	modified
crates/relune-render-html/src/metadata.rs	98.1%	+0.0%	modified
crates/relune-wasm/src/lib.rs	4.7%	-0.1%	modified
crates/relune-wasm/src/request.rs	98.5%	+0.1%	modified

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Reported by octocov