fix: cache ancestry headers across batches

[decofe]

Summary

• cache validated, RLP-encoded Tempo headers across overlapping ancestry-mode batches
• fetch only missing headers, typically the newly produced suffix
• use a fixed-capacity schnellru::LruMap capped at 262,144 headers (~150 MiB payload)
• add a mocked-RPC regression proving the second overlapping range fetches only one new header and the cache respects its capacity

Anchor reuse is intentionally deferred to a separate PR.

Validation

• cargo +nightly fmt --all -- --check
• cargo metadata --offline --no-deps --format-version 1
• git diff --check
• updated schnellru revision is running CI

Prompted by: @rkrasiuk

[0xalpharush]

maybe some changes in #692 worth pulling in