Add conflict tracing and pair-local repair

docs/frontier_trace.md

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+# Frontier Trace
+
+Generated at 2026-04-02T14:24:39-07:00 by `scripts/record_frontier_trace.py`.
+
+## example_07_large_scale_routing_no_warm_start
+
+Results: 10 valid / 10 reached / 10 total.
+
+| Net | Hotspots | Closed-Set | Hard Collision | Self Collision | Cost | Samples |
+| :-- | --: | --: | --: | --: | --: | --: |
+
+Prune totals by reason:
+
+- None
+
+Top traced hotspots by sample count:
+
+- None
+
+Per-net sampled reason/move breakdown:
+
+- None
+