…ction (#414)

## 🗒️ Description / Motivation

Ports the five `lean_block_proposal_*` observability metrics from
[leanSpec PR #753](leanEthereum/leanSpec#753)
into the ethlambda block builder.

These metrics give cross-client visibility into the block-proposal
attestation-selection path (`build_block`): how long each phase takes,
how many proposal builds run, how many child payloads are greedily
consumed, and how many distinct `AttestationData` / aggregated proofs
end up in the proposed block. They align with [zeam
#914](blockblaz/zeam#914
`getProposalAttestations` instrumentation and the leanSpec naming so the
[leanMetrics](https://github.com/leanEthereum/leanMetrics) dashboards
work across clients.

## What Changed

**`crates/blockchain/src/metrics.rs`** — five new metrics registered
with the existing `LazyLock` + `register_*!` pattern, a
`BLOCK_PROPOSAL_ATTESTATION_BUILD_PHASES` label constant, registration
in `init()`, public API functions, and a unit test:

| Metric | Type | Buckets / Labels |
|--------|------|------------------|
| `lean_block_proposal_attestation_build_phase_seconds` | HistogramVec |
`phase` = `select_payloads`, `compact`, `stf_simulate`; buckets
`0.001…8` |
| `lean_block_proposal_attestation_builds_total` | Counter | one per
proposal attempt |
| `lean_block_proposal_child_payloads_consumed_total` | Counter |
greedily-picked proofs before compaction |
| `lean_block_proposal_attestation_data_selected` | Histogram | buckets
`0, 1, 2, 4, 8, 16, 32` |
| `lean_block_proposal_aggregates_selected` | Histogram | buckets `0, 1,
2, 4, 8, 16, 32, 64, 128` |

**`crates/blockchain/src/block_builder.rs`** — instruments
`build_block`: times the `select_attestations`, `compact_attestations`,
and STF (`process_slots` + `process_block`) phases, and emits the
counters/histograms after a successful build.

**`docs/metrics.md`** — documents all five in the Block Production
Metrics table.

## Correctness / Behavior Guarantees

- **No behavior change.** Only metric observations were added around
existing logic; block contents, selection order, and the
state-transition path are untouched.
- **Architectural divergence from leanSpec, documented.** leanSpec
re-runs the STF inside a fixed-point loop and observes `stf_simulate`
per round. ethlambda projects justification/finalization incrementally
during selection and runs the STF exactly **once** at the end, so its
`stf_simulate` is a single observation per build. This is noted on the
`BLOCK_PROPOSAL_ATTESTATION_BUILD_PHASES` doc comment, consistent with
the upstream PR's own caveat that phase timings are not directly
comparable across clients.
- `attestation_data_selected` and `aggregates_selected` are observed
from the post-compaction body (one merged proof per distinct
`AttestationData`), matching the spec's intent.
- Metrics are only emitted on a **successful** build; a build that
errors out in the STF is already counted by the existing
`lean_block_building_failures_total`.

## Tests Added / Run

- New unit test
`metrics::tests::block_proposal_attestation_build_metrics_are_usable` —
verifies the phase metric registers and accepts every label in
`BLOCK_PROPOSAL_ATTESTATION_BUILD_PHASES`, and that the companion
counters/histograms are callable. Guards against drift between the label
constant and the strings passed at the `build_block` call sites.
- Existing `block_builder` tests (`build_block_*`,
`compact_attestations_*`, `extend_proofs_greedily_*`) now exercise the
new metric paths and pass unchanged.

Commands run:
- `make fmt` — clean
- `cargo clippy -p ethlambda-blockchain --all-targets -- -D warnings` —
clean
- `cargo test -p ethlambda-blockchain --lib` — 23 passing

## Related Issues / PRs

- Ports
[leanEthereum/leanSpec#753](leanEthereum/leanSpec#753)
- Related to
[blockblaz/zeam#914](blockblaz/zeam#914)
- Follows the metrics pattern from #406 (per-subnet attestation
aggregate coverage)

## ✅ Verification Checklist

- [x] Ran `make fmt` — clean
- [x] Ran `make lint` (clippy with `-D warnings`) — clean
- [ ] Ran `cargo test --workspace --release` — only
`ethlambda-blockchain` lib suite run (23 passing); full workspace
release run not yet executed