feat(frost/signing): expose ROAST-retry evidence counters via clientinfo

[mswilkison]

Summary

Process-wide cumulative counters for the three evidence categories
(overflow / reject / conflict), exposed through keep-core's
`clientinfo` registry so operators can observe per-category event
rates via the standard Prometheus scrape.

In default builds and unregistered-coordinator states, the
metrics-emitting recorder is bypassed entirely (the receive loops
use `attempt.NoOpRecorder`), so the counters stay at zero. Once
the ROAST-retry registry is populated and live signing flows
record evidence, the counters increment -- providing the
"do I have ROAST retry running?" smoke test from operator
dashboards.

Stacked on #3989 (AttemptContextHash enforcement).

What lands

File	Role
`roast_retry_metrics.go` (new, untagged)	Cumulative atomic counters; `RegisterRoastRetryMetrics(clientinfo.Registry)` registers Source functions under the `frost_roast_retry` application prefix; `metricsEmittingRecorder` wraps the bounded recorder and bumps the counter on each Record call.
`roast_retry_recorder.go` (modified)	`roastRetryRecorderForCollect` now wraps the bounded recorder with `newMetricsEmittingRecorder` when the registry is populated.

Metrics exposed

Via `clientinfo.Registry.ObserveApplicationSource`:

Metric name	Description
`frost_roast_retry_overflow_events_total`	Cumulative count of receive-channel overflow events
`frost_roast_retry_reject_events_total`	Cumulative count of validation-gate rejections (incl. `attempt_context_hash_mismatch` from #3989)
`frost_roast_retry_conflict_events_total`	Cumulative count of first-write-wins equivocation events

Test coverage (6 cases)

• Counters increment on Record* (different per-category counts)
• Snapshot delegates to inner recorder
• Nil inner falls back to NoOp without panicking
• Unregistered coordinator → NoOp recorder → no counter bumps
• Concurrent counter increments are race-safe (16 workers × 100 calls)
• RegisterRoastRetryMetrics(nil) is a no-op (defensive guard)

Operator wiring

The keep-core node's startup sequence should call:

```go
signing.RegisterRoastRetryMetrics(clientinfoRegistry)
```

alongside the existing registry observation calls. A follow-up to
`docs/development/frost-roast-retry-rollout.adoc` will document
this step.

Verification

Command	Result
`go build ./...`	clean
`go test ./pkg/frost/...`	pass (5 packages)
`go test -race ./pkg/frost/signing/...`	pass
`go test -tags 'frost_native frost_tbtc_signer frost_roast_retry' ./pkg/frost/...`	pass
`staticcheck -checks '-SA1019' ./pkg/frost/...`	silent
`go vet ./pkg/frost/...`	clean
`gofmt -l ./pkg/frost/signing/`	silent

Test plan

• CI green.
• Reviewer confirms the process-wide cumulative counter shape (alternative: per-session gauges, more granular but harder to query at a glance).
• Reviewer confirms the `frost_roast_retry` application prefix is acceptable (alternative: more specific prefix like `frost_roast_retry_evidence`).