Self-APM: remaining span coverage (loop.commit, logs_export steps, httpx, election)

Description

Migrated from followup.md §10 (v2 stream) at the 2026-08-14 tracker migration. Enrichment over the existing logs + counters; depends on the loop.emit span landing first, since propagation is the shared hard part.

Four candidates, in followup.md’s own priority order:

• loop.commit — the epoch-fenced Save (ConfigMap RMW) has only a checkpoint_fenced counter, so a slow or contended write and a fence trip have no timing. Lives in schedule/runner.go; child of the emit span or the tick depending on propagation.
• logs_export job lifecycle steps — the Portkey export is a multi-tick state machine (create → poll → download → page). Logs show coarse phase transitions, but where a slow export spends time (queued at Portkey vs download streaming) is not timed. Cross-tick correlation is the hard part: each step is a separate Collect, so the likely shape is a stored trace id in exportCursor plus span links.
• httpx request spans (upstream.request) nested under the tick — the upstream histogram gives per-target latency but not the causal parent, nor in-request detail (DNS, cross-host-redirect block, SSRF-guard reject). Wire it via the existing httpx.Observer seam or an otelhttp-style RoundTripper so httpx stays decoupled; that covers all source calls.
• coordinate.elect — lowest priority. Failover handoff duration is inferable from Lease leaseTransitions plus logs, and the lease-transition metric plus e2e already characterise it.

Treat the four as separately landable; a partial result is fine if the notes say which are done.

Acceptance Criteria

• #1 loop.commit span times the fenced Save and distinguishes fenced from clean
• #2 httpx request spans nest under loop.tick via the existing Observer seam, leaving httpx decoupled
• #3 logs_export per-step timing exists, or the cross-tick correlation approach is recorded as rejected with its reason
• #4 Any span or metric name added is reflected in ARCHITECTURE.md §11 and docs/DESIGN.md in the same change

Definition of Done

• #1 make gate
• #2 go test -tags acceptance ./internal/app/ (only if a §9 acceptance seam changed)