Layer time can be open-ended ("now") and granularity-aware

[BhattaraiSijan]

Closes #332 (part of epic #331)

Summary

Time-enabled layers could only declare fixed data start/end timestamps — but for growing collections the end is unknowable at authoring time, so every config went stale the day after it was written. A layer's dataStartTime/dataEndTime may now be a policy re-evaluated on demand: now, or now ± <ISO-8601 duration> (now - P1D, now + P5D). Concrete datetimes behave exactly as before.

Per veda-ui (our semantics standard), now resolves to the raw current moment — never rounded.

Core owns this vocabulary. The config keeps what the author wrote; core resolves it behind one provider, layers:getTemporalExtent, which answers { start, end } as plain ISO datetimes (or null) — for one layer by id, or for all layers keyed by UUID, in the same call shapes as layers:getCogCapabilities and layers:getBounds. It resolves at the moment of asking, so an open-ended now is always fresh. A plugin never sees a policy string and never imports a resolver.

Granularity (interval) and periodicity (isPeriodic) were originally declared on the bus contract here as carried facts. Nothing in this PR writes or reads them, so they land with #338's VEDA source instead, which is what actually produces them. The time block's index signature already allows both, so no consumer is blocked in the meantime.

Commits

1. The resolver — one pure function: policy string in, resolved ISO datetime out, null for anything unparseable so consumers keep their fallbacks and a bad value can never break the UI. Calendar-safe offset math (months/years via UTC date components, not milliseconds).
2. The Timeline consumes it — the per-layer bar previously hand-parsed dates with an inline 'now' special case on the end only; both ends now resolve.
3. Resolution moves into core — the resolver relocates to Basics/TimeControl_/layerTimePolicy.ts, beside the mission-level time parser; Layers_ registers layers:getTemporalExtent; the Timeline reads extents from it instead of resolving; _shared/time/ is removed with no re-export. The Layers tool's "set global time to layer extent" action goes through the same path — it previously did new Date("now") and produced NaN for a policy value.

Testing

• 15 unit specs with an injected clock: all three policy forms, sub-day and calendar-unit offsets, pass-through, every failure mode degrading to null. Full suite green (92 files, 1425 tests); typecheck clean.
• Verified live: an ongoing layer configured with end now draws its timeline bar reaching the current date (screenshot in Layer time can be open-ended ("now") and granularity-aware #332), while a closed-extent layer's bar correctly stops at its real end — and the now config needs no edits, ever.

Mission-level time (initialstart/initialend) keeps its existing seconds-offset syntax; unifying it with ISO durations is possible future work.

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