Implement the concrete external timer helper before v0.3.0 #110

The reopened coordinated v0.3.0 audit (fruwehq/determa-state-examples#21) found that timer schemas/vectors exist but no working installed timer helper is shipped. The approved implementation scope requires a minimal tested local reference even though application installation remains optional.

Implement the synchronous §23 version-1 timer helper with explicit setup/configuration, trusted clock and target/scope authorization. Persist closed requests and exact first results, signed-64-bit nanosecond deadlines, stable fire identities, monotonic revision/fences, due checks, cancellation, clock outage refusal and actual queue-bearing admission evidence. No background scheduler or hidden core clock. Timer records and their archive participant remain outside checkpoints; frozen/quarantined/retired ownership prevents firing. Supply a reviewed production bridge/installation registration for the existing timer conformance runner and meaningful restart/process-kill coverage.

Preserve unreleased 0.3.0, version-1 contracts and numeric machine format 1. No tag, publication or deployment. Ordinary checkpoint pushes and draft PR until required gates and independent exact-head LGTM. Depends on native-provider integration #109; archives/recovery integration follows in separate tracked work.