refactor: extract timer and meta logic from Hellog into isolated classes

[LMaxence]

Context

Follow-up from review of #55. As noted there:

The Hellog class is getting a bit bigger than what I'd like it to be [...] I'd like to move the timers and metas into isolated classes handling their logic and serde layers

The Hellog class now owns timer state (_timers map, time/timeEnd) and meta resolution/merging (_resolveMeta, child/log merge logic) inline. These should be extracted into dedicated classes that own their logic and serialization.

Proposed shape

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

Behavior must remain identical to today.