linera-storage: add in-memory caches for immutable DB reads

[ndr-ds]

Motivation

DbStorage pays a full Scylla/RocksDB round-trip on every call to four read methods that return permanently immutable data. On busy validators this contributes directly to Scylla read pressure (one node sustained ~7-8s p99 read spikes, with cache storms making it worse).

The four uncached paths:

• read_certificate_hashes_by_heights — (ChainId, BlockHeight) → CryptoHash
• read_event_block_heights — EventId → BlockHeight
• read_events_from_index — per-event bytes (keys still require a DB scan, but values now hit the cache)
• read_network_description — single NetworkDescription value

All four are permanently immutable once written. They are safe to cache indefinitely.

Proposal

Add three new caches to StorageCaches:

• block_hash_by_height: Arc<ValueCache<(ChainId, BlockHeight), CryptoHash>> — S3-FIFO bounded, consistent with the existing five caches
• event_block_height: Arc<ValueCache<EventId, BlockHeight>> — same
• network_description: Arc<OnceLock<NetworkDescription>> — single-value, zero overhead after first read

Read path: check-then-populate pattern (same as read_certificates_raw). Misses batch to DB; hits short-circuit. Write path: caches are populated in write_blobs_and_certificate at write time so write-then-read (the common path) is immediately cached.

read_events_from_index still scans DB for the key list (unavoidable), but now checks the existing event cache before bulk-reading values.

New CLI flags with conservative defaults (1 000 entries), overridden to larger values in the validator values.yaml:

• --block-hash-by-height-cache-size (validator: 100 000)
• --event-block-height-cache-size (validator: 50 000)

New Prometheus counters: READ_BLOCK_HASH_BY_HEIGHT_COUNTER, READ_EVENT_BLOCK_HEIGHT_COUNTER (labeled [CACHE]/[DB]). read_network_description gets [CACHE]/[DB] on the existing READ_NETWORK_DESCRIPTION_COUNTER.

Test Plan

• cargo clippy clean on --all-targets --all-features and --no-default-features
• Covered by existing integration tests exercising read_certificate_hashes_by_heights and read_events_from_index for the full round-trip

Release Plan

• These changes should be backported to the latest testnet branch, then be released in a validator hotfix.

Links

• Prerequisite backport (index to testnet_conway): docker: bundle pre-compiled Wasm contracts and linera-benchmark in linera-tests image #6393
• reviewer checklist