perf(watcher): reuse live root map for dispatch

[srinivasdrona]

Summary

• reuse the live watcher root index during file dispatch instead of rebuilding the root-to-adapter map from the registry on every poll/write re-dispatch
• reduces steady-state CPU in the watcher hot path during high-churn session file activity
• adds regression coverage for live-root dispatch

Benchmark

Measured on the same hot path (Watcher.adapterFor()):

• before: ~13.8 µs/op
• after: ~3.0 µs/op
• ~78% faster (~4.5x speedup)

Validation

• go test ./internal/watcher