[beinan]

Blob misses currently reacquire an exclusive store guard to refresh the resident dataset. A concurrent append/flush holds a shared guard, so one absent-ID lookup can wait behind the write and queue later blob hits behind itself.

Keep blob lookups under a shared guard and refresh cloned base snapshots around WAL fallback. After a complete WAL miss, check the latest base again so a row merged and drained between the initial base check and WAL discovery remains readable. Preserve resident and pinned versions, read caps, the base-hit fast path, and the bounded missing-file retries from #411.

Validation:

• Before the fix, the server regression times out while an independent shared guard is held.
• Before the fix, the deterministic core regression returns None for a row flushed before the read and merged during the base-to-WAL transition.
• After the fix: 31 blob tests (21 core, 10 server) and 4 point-lookup tests (2 core, 2 server) pass, including both regressions. cargo fmt --all -- --check also passes. Full CI remains in progress.
• Isolated Azure staging with a 291,593,422-byte same-table write, 53,248-byte blob reads, 32 pending generations, and concurrent hit/miss streams produced the maximum header latencies below. Each run used a fresh storage prefix; no OOM or container restart occurred. These are three sequential synthetic runs, not production p99 measurements.

Image/run	Hit maximum	Miss maximum
c9aa control	4.046 s	4.149 s
Patched run 1	0.0021 s	0.159 s
Patched run 2	0.0058 s	2.100 s

The second patched miss still took 2.1 s: this does not resolve every source of read latency. The runtime image IDs matched the immutable image built from this PR's exact head (768495e) in services build 2159160.

A separate isolated five-worker fixture with 100,586,357-byte payloads passed six status checks and four full payload comparisons during five concurrent writes. All three lookup types (base hit, WAL hit, absent ID) overlapped actual merges; the four writer shards reclaimed 32 generations. Maximum read header latency was 0.849 s, with no OOM/restarts. This is a bounded correctness run, not a soak test. All four fixture Pods were deleted and verified absent. Production has not been changed.