feat(editor): add playback speed multiplier for time-lapse export

[adisagar2003]

What

Adds a time-lapse speed multiplier (1x–16x) to the editor's Video tab. The exported video is
shortened by that factor: duration_out = duration_in / speed.

There's currently no way to speed a recording up, so a long build-along or refactor session can
only be exported at 1x.

How

The three exporters are hand-rolled AVAssetReader → FrameRenderer → AVAssetWriter loops where
output time, composition time and source time are the same value. This change decouples the
writer clock from the source clock and nothing else:

totalFrames = TimeLapse.frameCount(duration: trimDuration, fps: fps, speed: speed)
outputTime  = CMTime(frameIndex, timescale: fps)          // writer PTS, unchanged
sourceTime  = outputTime * speed                          // reader cursor + renderer clock

FrameRenderer keeps receiving source-domain composition time, so cursor metadata, zoom
keyframes, captions, camera/video/spotlight regions and transition durations all keep working
untouched — no changes in RegionRemapping, CompositionInstruction or FrameRenderer. That's
why this seam was chosen over composition.scaleTimeRange(...), which would desync those
domains. reader.timeRange still covers the full trim, so every source sample is still read.

Speed above 1x:

• forces the compositor path (checkNeedsCompositor) — passthrough can't resample;
• drops audio entirely: system, mic, mic noise reduction and click sounds. Sped-up audio isn't
  usable and time-scaling it was deliberately removed in v0.14.6 (22dc93c), so it isn't
  reintroduced here.

Preview applies the same multiplier to the player rate (multiplied onto the existing drift
ratios, not assigned over them) and mutes audio above 1x. The timeline stays in source seconds —
only the export gets shorter — and the panel shows the resulting length.

Tests

TimeLapse holds all the arithmetic and is covered by unit tests, including the two cases that
would otherwise bite: degenerate speeds (0 / negative / NaN → division by zero) and the last
frame never reading past the source duration.

⚠️ This adds the repo's first test target (ReframedTests, host-less so it doesn't launch the
app, Swift Testing) plus make test. Happy to split that into its own PR, or drop it, if you'd
rather keep the project test-free.

Verification status

Not compiled by me: I don't have Xcode on this machine, only the Command Line Tools, so
make build / make test never ran here. Flagging that up front rather than burying it. What
was verified:

• TimeLapse logic: unit tests run green through a standalone SPM harness, and mutation-checked
  (flipping ceil→floor and dropping the speed factor both make them fail).
• Every changed file passes swiftc -parse, and swift format is clean.

Still needs a real build + an export check:

• 4x on a ~30 s recording → ≈7.5 s and no audio stream (ffprobe), with cursor/zoom/captions
  still landing on the same on-screen events
• all three export paths (parallel, normal, GIF), plus a 1x regression (still passthrough)
• trim and/or a cut region combined with speed → cut points still land correctly
• preview at 8x/16x — AVPlayer can refuse rates above 2 for items that don't support fast
  forward, which would silently stall playback rather than error

Not included

• Per-range speed on the timeline (Screen-Studio style)
• Capture-time low-fps time-lapse
• Pitch-corrected sped-up audio