fix(query): prevent chain backtracking from discarding all paths on high fanout

[rocketman-code]

Summary

• --chain and --cut returned false negatives on high-fanout modules (e.g. src/dialogs.ts in workers-sdk with 25+ importers)
• Root cause: backtracking explosion guard broke the entire loop when partial paths exceeded max_chains * 2, discarding all incomplete paths before any reached entry
• Changed to a capped-expansion strategy that stops spawning new branches but still extends existing paths to completion

Test plan

• New unit test: chain_high_fanout_not_dropped (25-way fan-out graph)
• Verified --chain src/dialogs.ts on workers-sdk now returns 10 chains (was "not reachable")
• Verified --cut also works correctly
• Full test suite passes (464 tests)

Closes #172