Add AM625SiP inner-row microSD fanout repro

[AnasSarkiz]

Adding MMC1_DAT1 on inner-row B21 to the AM625SiP microSD fanout leaves the solver at 2/3 routed connections. A21/CMD and A22/DAT0 alone solve with no connectivity or copper DRC issues.

This PR captures the report and reproduction separately from the algorithm fix. It adds a numbered Cosmos debugger page with a control toggle, the captured native fanout input, and PCB-routing SVG snapshots. All 425 circular BGA pads and three testpoint obstacles are retained in both cases; the control removes only the B21 connection/bus. Debug-serializer markers representing undefined were decoded by omitting those properties.

Reproduce

bun install --frozen-lockfile
bun test tests/am625sip-microsd-fanout-repro.test.ts tests/am625sip-microsd-fanout-control.test.ts
bun run start

In Cosmos, open repro06-am625sip-microsd-fanout and run with B21 enabled, then disabled. The reproduction snapshot reports solved=false · best=2/3; the control solves 2/2. The failure test captures the current visual result without locking the solver to failure, following the existing northeast repro pattern.

Expected / actual

Expected: route all three connections to the shared boundary, with legal through-all vias and preserved pad geometry. Actual on v0.0.57 / b859939: B21 does not escape. The candidate search omits neighboring vacant BGA grid positions, including the diagonal position beside B21; a follow-up PR will test and fix that search.

Input: 0.50 mm pad pitch, 0.30 mm circular pads, 0.09 mm trace/clearance, 0.4572 mm via pad, 0.2032 mm drill, four layers, top/bottom escape, no blind/buried vias, 32 × 32 mm fanout boundary. Captured from the incremental AM625SiP circuit after adding B21 to v1.0.3. This isolates the native fanout phase; the circuit's global router remains Pipeline 9.

PCB snapshots

Failing three-signal case:

Working control:

Validation

Focused tests: 2 pass. Typecheck, format check, and Cosmos export pass. GitHub CI passes all 8 test shards, aggregate tests, typecheck, format, and the preview deployment. Local full suite: 158 pass; 2 existing tests timed out under concurrent benchmark load, and both pass unchanged when rerun individually (bus-layer-atomicity and footprinter-benchmark). Dataset31 baseline at the default 120-second sample deadline: 1/12 solved, 11 timeouts. No production solver code changes.

Follow-up fix: #158 (stacked on this PR).

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Project	Deployment	Actions	Updated
fanout-solver	Ready	Preview	Sep 5, 2026 12:57pm UTC