Add elapsed window anchor (run-relative distortions)

[boatmeme]

What & why

Generalizes where a distortion is anchored. Until now a window was always a wall-clock
time-of-day; this adds a second kind — elapsed — that pins a window to the run's own
t = 0 (the instant the Clock is constructed), measured in real elapsed time. This is the
engine seam the future Stopwatch/Countdown facades sit on (issue #4, step 2).

The shape of the change

Step 1 already made Clock consume windows through resolveAt(anchorMs), so the offset
integrator needs no changes — a new anchor kind is just another implementation of the same
tiny contract:

• DistortionWindow (new) — the resolveAt + clone contract; ResolvedWindow moves here.
• TimeWindow — now implements DistortionWindow; time-of-day logic untouched.
• ElapsedWindow (new) — resolveAt(anchor) = { anchor + startOffset, anchor + endOffset },
  in reference (real) time.
• RelativeTimeDistortion — first constructor arg generalized to
  DistortionAnchor = ClockTimeDescriptor | { elapsed: Duration }, branching on
  'elapsed' in anchor. Also drops the unused startClockTime field, decouples the
  getElapsedTimeInMillis default length to referenceDurationInMillis, and extracts a
  durationToMillis helper.

Design note

Elapsed windows are positioned on real elapsed time (only the display warps). This keeps
the integrator unchanged and makes "a countdown still hits zero on the real deadline" emerge for
free — placing windows on the warped timeline would make a window's bounds depend on the offset
it itself produces.

Backward compatibility

new ConstantTimeDilation({ hour: 1 }, { hours: 3 }, { hours: 6 }) is unchanged — a bare
descriptor still means time-of-day. The only addition is accepting { elapsed } in that slot.
Public runtime exports are unchanged (the DistortionAnchor type is exported;
ElapsedWindow/DistortionWindow stay internal).

Tests

• ElapsedWindow unit tests (additive resolution, independent clone).
• Clock-level elapsed-anchor tests: run-relative regardless of calendar date,
  polling-frequency independence, and a dilation repaid by a later compression netting to zero.
• Distortion-construction coverage ({ elapsed } → ElapsedWindow, bare descriptor →
  TimeWindow).
• Full suite green — 56 tests, tsc --noEmit and eslint clean.

Not in this PR

Stopwatch/Countdown facades (next step), the absolute anchor + multi-day spans (step 4),
daily recurrence, and eased distortions. No version bump (0.2.0 isn't shipped yet).

Refs #4

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