…pool metrics

Use parking_lot::RwLock instead of std::sync::RwLock for connection pool
metric recording. parking_lot::RwLock is faster for short-held locks as
it uses efficient spinning before parking, reducing tokio worker thread
blocking during metric recording.

This change helps reduce tokio threadpool contention when the connection
pool is under heavy load, as the metric recording locks are held for
only microseconds.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

With many subgraphs, chain_head_ptr() was querying the database on every
call, leading to connection pool saturation. This adds an adaptive cache
that learns optimal TTL from observed block frequency.

The cache uses EWMA to estimate block time and sets TTL to 1/4 of that
estimate (bounded by 20ms-2000ms). During warmup (first 5 blocks), it
uses the minimum TTL to avoid missing blocks on unknown chains.

New metrics:
- chain_head_ptr_cache_hits: cache hit counter
- chain_head_ptr_cache_misses: cache miss counter (DB queries)
- chain_head_ptr_cache_block_time_ms: estimated block time per chain

Safety escape hatch: set GRAPH_STORE_DISABLE_CHAIN_HEAD_PTR_CACHE=true
to revert to the previous uncached behavior.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Replace RwLock<MovingStats> with a lock-free AtomicMovingStats that uses
an atomic ring buffer with packed bins. Each bin packs epoch (32 bits),
count (32 bits), and duration_nanos (64 bits) into a single AtomicU128
for lock-free CAS updates.

This eliminates lock contention when many threads write concurrently
(every semaphore wait, connection checkout, query execution) while
reducing memory usage by 2x (4.8KB vs 9.6KB per stats instance).

Co-Authored-By: Claude Opus 4.5 <[email protected]>

…cks for active connections

Connections that were used within the last 30 seconds (configurable via
GRAPH_STORE_CONNECTION_VALIDATION_IDLE_SECS) now skip the SELECT 67 health
check during pool recycle. This reduces connection checkout latency from
~4ms to ~0ms for frequently-used connections while still validating idle
connections to detect stale database connections.

Co-Authored-By: Claude Opus 4.5 <[email protected]>