perf(state): measure mined block admission timing

[evan-forbes]

Summary

• measure the Tower readiness wait before buffered dispatch;
• carry the post-readiness mined-block request instant into the inner state
  service;
• measure buffered dispatch, prequeue checks, and queue-and-commit work
  separately;
• carry the writer-channel send instant with each queued block;
• measure all-block and mined-only writer queue duration when serialized
  processing starts.

Motivation

The earlier field profile measured 63.4 ms between semantic verification and
state queue admission. The writer-channel wait accounted for only about 0.3 ms
in the same data. These metrics measure both sides of that gap without relying
on differences between aggregate histogram quantiles.

Safety

This change records elapsed time on #748's existing mined-block admission path.
It does not change queue ownership, validation, branch selection, storage, or
publication.

The readiness timer starts before ready().await. The dispatch timestamp starts
after readiness succeeds and immediately before the buffered state call. The
writer timestamp starts immediately before the existing channel send.

Benchmark scope

This PR adds observability. It does not claim a runtime speedup. A local
before-and-after benchmark would not validate the change because the useful
result is the partition of the mined-block admission interval under fleet CPU
load. The state and consensus tests verify that the timestamps cross the
intended service boundaries. The next fleet run must compare the mined-only
readiness, dispatch, prequeue, queue-send, and writer-queue histograms on the
same block population.

Validation

• cargo test -p zakura-consensus -p zakura-state --lib --locked on the perf(state): measure contextual verification phases #782 stack tip
  • 210 consensus tests passed
  • 520 state tests passed and 3 tests remained ignored
• cargo check -p zakura-consensus -p zakura-state --all-targets --locked
• cargo clippy -p zakura-state -p zakura-consensus --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• ./scripts/changelog.py check

This PR adds docs/changelog/unreleased/781.md with an internal-only marker.

Stack

• Base and parent: feat(mining): advertise admitted blocks optimistically #748.
• Child: perf(state): measure contextual verification phases #782.
• This branch does not contain feat(mining): submit prepared blocks by solved header #754, perf(consensus): accelerate block verification #776, perf(state): avoid contextual UTXO copies #779, perf(state): share contextual snapshot maps #780, or feat(consensus): relay expected-work blocks before contextual commit #773.