feat(pivot): port clockreceiver off internal selftel + lifecycle

[trilamsr]

Summary

• Port components/receivers/clockreceiver off internal/selftelemetry + internal/runtime/lifecycle. Helpers travel as siblings (selftel.go + lifecycle.go) co-located with the receiver, per RFC-0013 §migration PR-B (mechanical mirror of PR-B1 feat(pivot): PR-B1 — port nccl_fr off internal selftel + lifecycle #184 for nccl_fr).
• Receiver-scoped meter is acquired from set.Telemetry.MeterProvider. Instrumentation scope name = the receiver's Go import path (github.com/tracecoreai/tracecore/components/receivers/clockreceiver).
• Metric names + label shape preserved (tracecore.receiver.{errors_total,emissions_total,collection_latency_seconds,degraded_seconds_total,last_activity_unix_seconds} with component_id + kind). The cmd/tracecore integration tests that assert these names by string still pass.
• Lifecycle sibling is slimmer than the internal helper: drops Add() + the post-Shutdown silent-no-op path because clockreceiver is single-source (same shape as the nccl_fr sibling). panicCallback(any) signature preserved so the panic → IncError(kindPanic) + SetDegraded(true) wiring is untouched.
• kind enum is trimmed to the two values clockreceiver actually emits (downstream, panic) — no speculative kinds.

Why port-before-delete

clockreceiver is scheduled for deletion in PR-K, so a fair question is "why port it at all?" Three reasons:

1. Unblocks PR-F earlier. The internal/selftelemetry + internal/runtime/lifecycle deletes in PR-F are blocked on every consumer porting off or being deleted. Porting clockreceiver here means PR-F can land before PR-K, instead of waiting on the kernelevents / stdoutexporter / dcgm porting wave plus the clockreceiver delete.
2. Mechanical mirror of PR-B1. This diff is a sibling-rename of PR feat(pivot): PR-B1 — port nccl_fr off internal selftel + lifecycle #184 — the review surface is the trimmed kind enum + the package-name swap. No new design choices.
3. Anchors the pattern. clockreceiver is the "canonical example receiver" per its package doc; even briefly carrying the wrong shape would signal that the migration is optional.

Partial credit toward the internal/selftelemetry + internal/runtime/lifecycle deletes scheduled for PR-F.

Test plan

• make check (gofumpt, golangci-lint, go vet, mod verify) — 0 issues.
• go test ./... repo-wide — green for the clockreceiver and cmd/tracecore packages, which assert tracecore_receiver_* metric names. (kernelevents TestReceiver_SLIBudget shows a known p99-latency overshoot under full-suite load; passes in isolation with -count=3; unrelated to this PR.)
• New tests: selftel_test.go (6 tests — noop safety, nil-MP, errors_total + kind, emissions_total, scope-name pin, init_errors_total + factory-fallback seam), lifecycle_test.go (5 tests — happy path, idempotent Start/Shutdown, panic recovery, shutdown-deadline).
• 9-lens adversarial review (sibling-impl drift, lifecycle drift, test correctness, cardinality, doc-rot, ctx usage, race-safety, helper duplication, scope creep) — no findings. Bonus catch: r.logger now falls back to slog.Default() when set.Telemetry.Logger is nil; the original would have nil-dereferenced.
• grep "internal/selftelemetry\|internal/runtime/lifecycle" components/receivers/clockreceiver/*.go — only doc-comment mentions remain (no imports).

Follow-ups

• PR-F: delete internal/{componentstatus,selftelemetry,runtime/lifecycle} once every remaining consumer (stdoutexporter, dcgm, kernelevents, containerstdout) has ported or been deleted.
• PR-K: delete clockreceiver itself (replaced by hostmetricsreceiver heartbeat).

Binding doc: docs/rfcs/0013-distro-first-pivot.md §migration PR-B.

Release notes

[CHANGE] clockreceiver migrates its self-telemetry + lifecycle helpers to co-located sibling files; emitted metric names + labels are unchanged but the OTel instrumentation scope is now the receiver's Go import path.