feat(etl): Wave 2C — filesystem watcher (watchfiles, ~400ms sync)

[0bserver07]

Summary

• New stackunderflow/etl/watcher.py — Rust-backed watchfiles daemon thread that watches every default-on adapter's source paths. On any change → ingest the new bytes via the existing ingest_file writer (scoped to the changed file, not a full enumerate sweep) → lazily run the per-provider Wave 2A normalizer → lazily call refresh_all_marts(conn) from Wave 2B. Debounced 200ms to coalesce JSONL append bursts.
• New BaseAdapter.watch_paths() Protocol method (default []). Concrete implementations on the four default-on adapters: claude (~/.claude/projects + ~/.claude-{opus,sonnet,haiku,glm} variants if present), codex (~/.codex/sessions), cursor (the vscdb file directly — watchfiles fires on byte change), cline-family (the globalStorage/<extension>/tasks root, inherited by KiloCode + Roo Code through the shared base class).
• New CLI flag stackunderflow start --no-watcher for headless / debugging runs (also honours STACKUNDERFLOW_DISABLE_WATCHER=1).
• Wave 2A (etl.normalize) and Wave 2B (etl.watermark.refresh_all_marts) are imported lazily with ImportError fallback — the watcher works as a pure incremental-ingest trigger today, and the normalize + marts steps activate automatically when those waves land. No coupling to in-flight code.

Smoke test

Run against the user's real ~/.claude/projects (~150 projects, ~17K msgs):

00:29:20.237 stackunderflow.etl.watcher etl.watcher: refreshed marts in 72ms — 1 events
=== SMOKE TEST RESULT ===
  end-to-end latency (append → row in DB): 155ms

Spec target: ~400ms. Actual: 155ms, well under target.

Test plan

• pytest tests/ -q clean: 1341 → 1352 passing (+11), 2 skipped.
• ruff check stackunderflow/etl/ tests/stackunderflow/etl/ passes (no new warnings on modified files; pre-existing UP017/UP038 issues on cursor.py / cline.py left untouched per the "Wave 2C only" scope).
• Smoke test against real ~/.claude/projects: append → DB row in 155ms.
• 11 new tests in tests/stackunderflow/etl/:
  • test_adapter_watch_paths.py (4): every default-on adapter exposes the right roots.
  • test_watcher.py (7): watch_paths_for filters missing roots; _adapter_for_path prefix-matching; idle-handle when no roots; append → refresh within 2s; burst-of-5 collapses into one cycle (debounce); WatcherHandle.stop() joins within timeout.

Constraints honored

• No version bump (still v0.6.1).
• 1341 backend tests still pass.
• Watcher never crashes on a bad event — every step wrapped in broad except.
• Builds to the spec, not to in-flight Wave 2A/2B branches (lazy imports + ImportError fallback).

Platform notes

Verified on macOS (FSEvents-driven). watchfiles claims cross-platform but the Cline / Cursor paths are macOS-only in v1 anyway, so cross-platform parity falls out of the existing adapter constraints, not the watcher.