Add absolute anchor and Clock-level timezone

[boatmeme]

What & why

The third and final window anchor from issue #4. { absolute } pins a distortion to a
fixed real-world instant, independent of when the Clock was constructed — and, since
it's plain instant arithmetic, it delivers true multi-day spans (an arbitrarily long
referenceDuration like { hours: 72 }, with none of the ~24h overnight-wrap limit a
time-of-day window has).

Resolving an absolute wall-clock start needs a timezone, so the Clock gains one — which
also closes a latent gap: time-of-day windows were silently UTC-only.

API

• { absolute: Date | AbsoluteDescriptor } added to DistortionAnchor (bare descriptor
  still defaults to time-of-day; { elapsed } unchanged). Date = an instant;
  AbsoluteDescriptor = { year, month, day, hour?, ... } resolved in the clock's zone.
• new Clock(distortions, { timeZone }) — read-only, default 'UTC', plus a timeZone
  getter. Used by time-of-day and absolute windows alike.
• Exports the AbsoluteDescriptor type.

How it fits

The resolveAt(anchorMs, timeZone?) extension to the window contract absorbed everything —
the offset integrator is unchanged. TimeWindow honors the passed zone (construction-zone
fallback for standalone use); ElapsedWindow ignores it (run-relative).

Design note

The zone is immutable — to run in a different zone, build a new Clock. Mid-run zone changes
are deliberately out of scope: the offset is path-dependent and time-of-day windows refire/skip
ambiguously when local time jumps, so that needs an injectable run-anchor/offset (seeded
successor clock) plus a refire policy — a separable follow-up.

Tests / docs

• New AbsoluteWindow, AbsoluteAnchor (incl. a 3-day span integrated once across a sparse
  read), and ClockTimezone (same window, different instants per zone) suites — 72 tests,
  coverage thresholds met, tsc + eslint clean.
• README: { absolute } anchor (multi-day example) and the { timeZone } option.

Not in this PR

Mid-run zone rebasing, daily recurrence, eased distortions, calendar-aware durations. No
version bump (0.2.0 isn't shipped yet).

Refs #4

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