fix(cli): snapshot picks the frame through a rate lane and up to 10x

[miguel-heygen]

What changes

hyperframes snapshot picks the source frame for each video clip. It clamped a clip's constant playback rate at 5 and ignored a speed-ramp rate lane, so a clip at 8x, or one with a ramp, showed the wrong frame in a snapshot while the preview and the render were right.

It now reads the rate through the runtime's own reader (a rate lane wins, then data-playback-rate clamped 0.1 to 10, then the browser default) and maps time with the shared sourceTimeAt / timeAtSourceTime.

Verification

• snapshot.test.ts: 38 passed on the devbox (exit 0). New tests cover rate 8, an invalid authored rate falling back to the browser default, and a rate lane mapping 2 s of clip to 3.641 s of source.
• tsc --noEmit in packages/cli: exit 0.
• Independent review found one regression (invalid authored rate ignoring the browser default); fixed and tested. Not exercised: a real snapshot render of a ramped clip.

Audited	Trusting	Not exercised
reader parity with the runtime on 11 attribute cases, every consumer of the rate in snapshot.ts, duration and loop arithmetic	core's readElementRateSpec clamp	real browser defaultPlaybackRate != 1, startup time of the core import

[miguel-heygen]

Folded into #4153 (one speed-ramp feature, one PR).