perf(watcher): reuse live root map for dispatch

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to SuperBased Observer are documented here.
 
 ## [Unreleased]
 
+### Changed
+
+- **perf(watcher): reuse the live root map during file dispatch** — avoids
+  re-scanning every detected adapter on each poll/write re-dispatch, reducing
+  steady-state watcher CPU during high-churn session files.
+
 ## [1.32.0] — 2026-08-26
 
 ### Fixed

internal/watcher/watcher.go

@@ -760,6 +760,9 @@ func (w *Watcher) pollCursors(ctx context.Context) error {
 // The registry is still queried per call so dynamically-added
 // adapters appear without restarting Watch.
 func (w *Watcher) adapterFor(path string) adapter.Adapter {
+	if byRoot := w.snapshotByRoot(); len(byRoot) > 0 {
+		return adapterForPath(byRoot, path)
+	}
 	byRoot := map[string]adapter.Adapter{}
 	for _, a := range w.registry.Detected(w.allow) {
 		for _, root := range a.WatchPaths() {

internal/watcher/watcher_test.go

@@ -396,6 +396,18 @@ func TestNewClampsNegativePollInterval(t *testing.T) {
 	}
 }
 
+func TestAdapterForUsesLiveRootIndex(t *testing.T) {
+	t.Parallel()
+	w, _, root := setup(t)
+	stub := &stubRetryAdapter{name: "stub", root: root}
+	w.byRoot = map[string]adapter.Adapter{root: stub}
+
+	got := w.adapterFor(filepath.Join(root, "nested", "value.stub"))
+	if got != stub {
+		t.Fatalf("adapterFor used registry scan instead of live root index: got %T want %T", got, stub)
+	}
+}
+
 // TestCodexShortSessionMultiPassIngest reproduces the live failure mode
 // reported 2026-05-06: a short ChatGPT-auth Codex rollout grows in three
 // chunks (leading prompts, then function_call + token_count, then final