perf(daemon): shard TraceIngressState with DashMap, per-root normalizers, bounded connection pool

Three architectural improvements to the trace2 ingestion pipeline:

1. Replace Mutex<TraceIngressState> with 15 independent DashMap/DashSet fields
   - Each field (root_worktrees, root_families, root_argv, root_mutating, etc.)
     is now a dashmap::DashMap<String, V> or dashmap::DashSet<String>
   - Per-root events no longer serialize on a single global mutex; concurrent
     connections to different roots run without blocking each other
   - Readonly fast path is now fully lock-free: DashMap shard lookups replace
     global Mutex acquisition for the 40+/sec readonly events from IDEs like Zed
   - Fold `mark_trace_root_activity` into the existing lock acquisition in the
     mutating path, eliminating one full Mutex cycle per mutating event
   - Add `dashmap = "6"` dependency

2. Per-root normalizer instances (dashmap::DashMap<String, Arc<AsyncMutex<...>>>)
   - Replace single global AsyncMutex<TraceNormalizer> with a per-root map
   - Normalizer state is now scoped to each root SID; entries are cleaned up
     when clear_trace_root_tracking removes the root
   - Enables future concurrent ingest workers per root without further refactor

3. Bounded trace connection thread pool
   - Add a hard ceiling (MAX_CONCURRENT_TRACE_CONNECTIONS=512) on simultaneous
     connection handler threads
   - Excess connections are dropped with a log message rather than spawning
     unbounded OS threads (prevents memory exhaustion and scheduler overload)
   - Active count tracked with a shared AtomicUsize; thread spawn/exit
     are O(1) atomic operations

All 1420 tests pass. Benchmarks: readonly_flood/zed_mixed_1000_events unchanged
(p=0.99; concurrency benefit not captured by single-threaded microbenchmark).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: svarlamov

Cargo.lock

@@ -38,6 +38,7 @@ gix-index = "0.48.0"
 regex = "1.12"
 toml = "0.9"
 unicode-normalization = "0.1"
+dashmap = "6"
 
 [target.'cfg(windows)'.dependencies]
 named_pipe = "0.4.1"