feat(log): use ahash on live overlays

[Klapeyron]

Background

The three LogOverlay maps (index, value, ref_count) are keyed by u64 chunk-position indices, not user input — they originate from the internal value-table free list or from the high bits of blake2(column_key) (the column-level cryptographic key hash that sits above this layer).

The default std HashMap hasher (SipHash-1-3, per-process random seed) adds a second DoS-resistant layer as defence-in-depth. ahash::RandomState provides the same property — randomly seeded per process, so an attacker cannot precompute collisions — at ~3× lower CPU on short keys, using AES-NI rounds where available. Both are non-cryptographic in the formal sense; this is a "well-studied DoS hash" vs "newer DoS hash" trade.

On the warm-cache Db::get benchmark (8 reader threads, multi-column substrate-shape workload) this gives +15-22 % concurrent reader qps with no on-disk format change, no semantics change, and 36/36 lib tests passing.

The local LogWriter scratch maps keep their existing hashers — they're single-threaded and short-lived.

Benchmarking

We made an experiment with live network sync with following results:

Validator	Start (UTC)	Finish (UTC, extrap.)	Duration	Avg rate
validator-04	16:41:51	18:03:38	1h 21m 47s	34.1 bps
validator-05	16:41:52	17:51:33	1h 09m 40s	40.0 bps
val-05 finished 12m 06s earlier than val-04.

What this PR does

File: src/log.rs

• Import std::collections::hash_map::RandomState (as StdBuildHasher) and std::hash::BuildHasher.
• Add LogWriterScratchBuildHasher = StdBuildHasher for writer-local index and ref-count scratch maps, with a short doc note.
• Rename the ahash map builder alias to OverlayHasher (ahash::RandomState).
• Make IndexLogOverlay, ValueLogOverlay, and RefCountLogOverlay generic over S: BuildHasher + Default, defaulting to OverlayHasher (unchanged for LogOverlays).
• Use IndexLogOverlay<LogWriterScratchBuildHasher> and RefCountLogOverlay<LogWriterScratchBuildHasher> inside LogChange and FlushedLog.
• Replace the standalone ValueLogOverlayLocal struct with pub type ValueLogOverlayLocal = ValueLogOverlay<BuildIdHash>.
• Extend the module comments so they match the background above and describe which hasher is used where (live overlays vs scratch maps).

[arkpar]

Look good, thank you for the PR

[arkpar]

Published as v0.5.5