[bpacholek]

Closes #183

"About N s left" kept going up during exports. There were two causes.

1. The progress was wrong. Transformer's progress is the average of every sequence's. Clipra's composition has one video sequence (the canvas the layer compositor draws on) and one sound sequence per layer with sound, and the sound is done long before the picture. On the demo project the export stood at 50% after 10 s, then crawled. ExportRenderer now takes progress from the frames the compositor has drawn (LayerCompositor.Listener), so the percent and "Frame N of M" are the video's.

2. The estimate was by time. It assumed every second of the timeline costs the same. ExportWork (core:model) weighs each tenth of a second of the timeline:

• 1 for drawing and encoding the frame;
• each visible video's source pixels against the output's, times its speed;
• 0.1 for a photo, 0.05 for text, 0.3 for an adjustment.

TimeLeft gives the work still to do at the pace of the work done so far.

How the estimator was chosen. I logged a full export of the demo project on the emulator (a 535 s export: 4K HEVC street clips, photos, a 4K talk, screen recordings as overlays) and replayed it through several estimators against the true time left:

Estimator	Median error	Worst error
By time, as before (with the right progress)	27%	113%
Pace of the work so far (this PR)	9%	27%
Pace of the work over the last 10 s	32%	unbounded
Pace over the last 30 s	24%	1024%

The pace over recent seconds fails because, on the emulator, a decoder starting at a clip change held the frames for about ten seconds, twice. The estimate then went to hours.

Checked on the emulator. With this build, the same export showed (estimate / true seconds left): 512/478, 413/449, 336/335, 282/223, 175/166, 129/108, 84/80, 28/27. From 30 s in, the median error was 8%. The worst was 68%, in the first half minute.

Tests: ExportWorkTest:

• the work is in the video, not the photos, and an export at that pace is right from 10% on;
• a 10 s stall at a clip change keeps it within 30% (28% at worst, just after it);
• nothing is shown before 2%.

ExportProgress looks as before.

🤖 Generated with Claude Code