refactor: extract Timer and Meta into isolated classes

[LMaxence]

Summary

Follow-up to #55. Extracts the timer and meta responsibilities out of the growing Hellog class into dedicated classes that own their logic and serialization, per the review note on #55.

• lib/timer.ts → HellogTimer — owns the Map<string, bigint> timer store, start(label), end(label): number | undefined (one-shot), and a static format(label, ms) for duration serialization.
• lib/meta.ts → HellogMeta — owns meta resolution and parent/child merging via resolve(), merge(extra), child(extra), replacing Hellog._resolveMeta and the inline merge logic in child / _log.
• Hellog now delegates to both. Public behavior (child meta merging, dynamic meta, timer durations, durationMs) is unchanged.
• Both classes exported from lib/index.ts; added lib/timer.spec.ts and lib/meta.spec.ts.

Closes #56.

Notes

Stacked on #55 — this branch builds on feat/child-loggers-dx, so until #55 merges the diff against main also shows #55's changes. Merge after #55.

Test plan

• npm run build — clean under @tsconfig/strictest
• npm test — 45/45 pass (32 existing + 13 new timer/meta specs)
• npm run lint — oxlint clean
• npm run format:check — oxfmt clean

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