metric: replace 10 duplicated bucket literals with leanSpec-named constants (#303)

[mananuf]

Summary

Ten of gean's 21 histograms in `node/metrics.go` had inline `[]float64` bucket literals whose values already matched one of the four named bucket constants leanSpec defines in `subspecs/metrics/registry.py:36-45`. Same bucket boundaries copy-pasted 2-6 times per latency shape. Replaces them with 4 named package-private vars that mirror the spec.

Closes #303.

What changed

Single file, `node/metrics.go`: +25 / -15.

Added (4 vars at top of histograms block)

```go
// Bucket sets mirror leanSpec's named constants in
// lean_spec/subspecs/metrics/registry.py:36-45. Defined here once so the
// histograms that share these shapes don't each carry a duplicated literal
// that can drift out of spec alignment silently. Histograms whose latency
// shape doesn't fit any of the four spec constants use their own hand-rolled
// bucket array inline and note that fact in a comment.
var (
forkChoiceBlockBuckets = []float64{0.005, 0.01, 0.025, 0.05, 0.1, 1, 1.25, 1.5, 2, 4}
attestationValidationBuckets = []float64{0.005, 0.01, 0.025, 0.05, 0.1, 1}
stateTransitionBuckets = []float64{0.25, 0.5, 0.75, 1, 1.25, 1.5, 2, 2.5, 3, 4}
reorgDepthBuckets = []float64{1, 2, 3, 5, 7, 10, 20, 30, 50, 100}
)
```

Replaced (10 histograms)

Histogram	Now uses
`lean_fork_choice_block_processing_time_seconds`	`forkChoiceBlockBuckets`
`lean_attestation_validation_time_seconds`	`attestationValidationBuckets`
`lean_committee_signatures_aggregation_time_seconds`	`stateTransitionBuckets`
`lean_pq_sig_attestation_signing_time_seconds`	`attestationValidationBuckets`
`lean_pq_sig_attestation_verification_time_seconds`	`attestationValidationBuckets`
`lean_fork_choice_reorg_depth`	`reorgDepthBuckets`
`lean_state_transition_time_seconds`	`stateTransitionBuckets`
`lean_state_transition_slots_processing_time_seconds`	`attestationValidationBuckets`
`lean_state_transition_block_processing_time_seconds`	`attestationValidationBuckets`
`lean_state_transition_attestations_processing_time_seconds`	`attestationValidationBuckets`

Out of scope (11 histograms left hand-rolled)

These either don't measure time at all (byte sizes, payload counts, coverage ratios) or measure a time shape no spec bucket constant covers (`block_building`, `attestations_production`, aggregated-sig building/verification, full-block sig verification). Left alone.

Zero behavior change

Each replaced literal had the same values byte-for-byte as the named constant it now points to. Verified by reading the leanSpec file side-by-side and by:

• `go test -count=1 ./node/...` green
• No new buckets, no removed buckets, no reordering

A `/metrics` scrape post-merge will produce byte-identical bucket boundaries for the 10 affected histograms.

Spec compliance

Addresses the standing ⚠️ from the previous `/spec-compliant devnet3 head` audit:

"Histogram bucket boundaries should be cross-checked vs spec constants. Possible drift — worth verifying."

After this PR + #301, all gean histograms whose latency shape matches a spec-named bucket constant traceably reference it. The 11 hand-rolled ones are honestly hand-rolled (no spec constant covers them) and don't pretend otherwise.

Lean-review

Single subtraction-oriented pass during implementation:

• `duplicates-existing-util`: ✅ collapses 10 duplicated bucket arrays into 4 named refs
• `comment-bloat`: header comment is 6 lines but earns its keep (explains the spec cross-reference + the policy for hand-rolled histograms — both non-obvious to a future reader)
• `premature-abstraction`: 4 named vars used 2-6 times each; threshold for naming clearly met
• All other categories: 0 findings

Related

• Stacked on PR perf(xmss): adopt [profile.multisig-release] + gean-glue shim (#298) #299 (`perf/multisig-release-profile`) — base branch.
• Closes metric: replace 10 duplicated histogram bucket literals with leanSpec-named bucket constants #303
• Builds on metric: align lean_committee_signatures_aggregation_time_seconds buckets with leanSpec STATE_TRANSITION_BUCKETS #301 (which adopted `STATE_TRANSITION_BUCKETS` values for one histogram); this PR extracts those values to a named constant + applies the same alignment to 9 other histograms.