feat: add replay stage observability

[allora2026]

Summary

Adds Phase 1 replay-stage observability to @flowcore/data-pump without changing replay algorithms or concurrency.

Measurement added

• fetch duration/result and returned event counts
• handler duration/result with bounded batch-size buckets
• acknowledgement duration/result
• checkpoint duration/result
• idle duration with finite reasons
• duration buckets covering slow replays through 120 seconds

Existing metric names, exports, and private/default registry behavior are preserved. New result/reason/batch labels use finite enumerations.

This PR intentionally does not change buffer algorithms, acknowledgement membership checks, bucket traversal, prefetch, or in-flight concurrency. It establishes the baseline for the next optimization PR.

Verification

• bun test — 61 passed, 0 failed
• bun run lint — pass
• bun run typecheck — pass
• bun run format:check — pass
• bun run build — pass
• git diff --check — pass
• class-based state-manager receiver regression covered
• independent spec review — pass
• independent code-quality/security review — approved

Research

• https://usable.dev/dashboard/workspaces/60c10ca2-4115-4c1a-b6d7-04ac39fd3938/fragments/d8309cc9-d4ad-4930-b9ab-ffd702a821ad
• https://usable.dev/dashboard/workspaces/60c10ca2-4115-4c1a-b6d7-04ac39fd3938/fragments/3ac9bdcc-2e35-4e72-9078-8bfefee9a8e6