v0.5.0 — idle speed-up

New: --idle-speedup (or idleSpeedup: true | { maxIdleMs, speed }) fast-forwards the boring parts — spinners, slow loads, long silent waits — at 3x by default. The design guarantee: narration playback and click choreography always play at 1x; only narration-free spans longer than maxIdleMs compress, with gentle 1x margins at each edge. Audio offsets, subtitle cues, and --zoom windows all remap through the same time map, in the same single ffmpeg pass. In the e2e suite a tutorial with a 4s silent wait renders 10.5s → 7.5s with narration still in sync. Closes #3.