Anchor distortion windows at construction (fix cross-midnight + sparse-read bugs)

[boatmeme]

What & why

Distortion windows were hardwired to re-resolve to "today" on every read
(TimeWindow → ClockTime.forTodayInMillis), while Clock._offset accumulates on a
single continuous timeline. Across a calendar boundary these two models conflict, which
produced two latent correctness bugs — one already pinned in the suite as a known
limitation with a regression target:

• Cross-midnight drop-out — an overnight window (e.g. 23:00→03:00) stopped
  distorting after midnight, because its start re-resolved to tonight's future
  occurrence. This is precisely the README's bedside-clock use case.
• Sparse-read gap — read before a window → idle through it (even for days) → read
  after, and only the read-time "today" instance was integrated; the whole excursion was
  silently skipped.

The fix

Pin each window once, at Clock construction, to a single fixed [start, end)
occurrence (one-shot semantics) rather than re-resolving on every read:

• Clock captures a _runAnchor at construction and resolves every window against it.
• TimeWindow.resolveAt(anchorMs) returns the occurrence containing the anchor, else the
  next upcoming one — with prior-day look-back so a midnight-wrapping window's current
  occurrence (which may have started yesterday) is found. Legacy today-resolving getters
  are kept for backward compatibility.
• ClockTime.forDateInMillis resolves a wall-clock time on an arbitrary date.
• Adds a recurrence: 'none' | 'daily' seam ('none' implemented), establishing the
  anchor abstraction that the deferred multi-day / stopwatch / countdown modes build on.

Tests

• The two previously-pinned "known limitation" cases now assert the fixed behavior
  (distortion continues across midnight).
• New regression coverage: cross-midnight continuity, and sparse reads integrating a
  fully-elapsed window exactly once.
• Full pre-existing suite passes unchanged — 48/48 green, tsc --noEmit and eslint
  clean.

Notes

• New user-observable behavior: a window now pins to its occurrence as of when the
  Clock is constructed — build the clock at/before the window you intend to bend.
  Documented in a new README "How windows are scheduled" section.
• Version bumped to 0.2.0 (minor — behavior change on a 0.x line); also corrects
  pre-existing package-lock.json version drift (0.0.1 → 0.2.0).

Refs #4

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