[8.1] Fork-resolution gauges and fork-boundary log lines

[rootwarp]

Plan reference: plan/glamsterdam-2026-09-05/issues/08-phase-8-network-soak-and-observability.md · Issue 8.1

Description

Expose the resolved current fork and the next activation epoch as metrics, and log one boundary event per fork change, so a pre-fork activation is never unmonitored (P2-1).

Implementation Notes

• Add ForkSchedule::next_activation(&self, epoch) -> Option<(ForkName, Epoch)> beside entries() (crates/eth-types/src/fork.rs:67); it returns None for a far-future sentinel entry.
• Declare in crates/rvc/src/metrics.rs (ARCH-6h owner, not metrics/src/definitions.rs): rvc_fork_current_id (IntGauge, ForkName::id()) and rvc_fork_next_activation_epoch (IntGaugeVec, label fork); force both in metrics::init(). Declare through metrics::define_int_gauge / define_int_gauge_vec (crates/metrics/src/lib.rs:149,156) — both are in NAME_CTORS (metric_name_stability.rs:28-29); any other constructor is invisible to metric_name_set_is_unchanged's source scan and the name ships unpinned while CI stays green.
• Update per slot in DutyOrchestrator::run (crates/rvc/src/orchestrator/coordinator/mod.rs:393) at :411, where current_epoch is already computed and self.config.fork_schedule is in scope — not in AttestationService::process_slot (crates/rvc/src/orchestrator/attestation.rs:123), which returns NoDutiesForSlot early at :163.
• Sentinel: omit the series (never create the child; remove_label_values when it becomes sentinel) — u64::MAX does not fit i64 and a fabricated number would alert wrongly.
• Log at info only when the resolved fork changes; use crates/observability/src/pubkey.rs helpers if any key appears, so no_crypto_logging_paths.rs stays green.

Acceptance Criteria

• test_next_activation_skips_far_future_sentinel — a schedule with gloas_fork_epoch = u64::MAX returns None.
• test_next_activation_epoch_series_absent_at_sentinel — registry gather contains no rvc_fork_next_activation_epoch sample.
• test_fork_current_id_tracks_resolved_fork — gauge equals ForkName::from_epoch(...).id() across an activation epoch.
• test_fork_boundary_logged_once_per_change — N slots inside one fork emit one boundary line.
• EXPECTED_METRIC_NAMES gains both names (sorted) and the pinned count assertion (crates/metrics/tests/metric_name_stability.rs:186-188) moves by exactly +2 from whatever it reads at branch time, with an operator-facing note. No absolute target (D4).

Testing Notes

unit tests in #[cfg(test)] at each file's bottom; existing create_test_fork_schedule helpers supply schedules; tracing-test captures the log assertion.

Points: 3 · Type: feature · Priority: P0 · Scope: 1.5 days · Blocked by: P2 (ForkName::Gloas), P7 · Blocks: #333, #337