Version 1.3.18

[maboa]

Two caption fixes, both reported from real use.

• A caption from the previous document stays painted over newly imported media #636 — a caption from the previous document no longer stays painted over newly imported media. A caption swap is written before the new media arrives: the VTT/SRT import sets the track from its own FileReader while the media is still loading from another, so the flush ran against the outgoing media. When the new media loaded, the picture was re-composited but the native caption overlay was not, and the old cue sat there unchanging while the real captions came and went above it. The flush is repeated once each time the media actually changes. Part 4 of Captions from the previous media aren't cleared when loading a file from Recents #356/Occassionally two sets of captions are shown contemporarily #287, the one the import path fell through.
• Caption and HTML downloads ignore the project name #637 — caption and HTML downloads take the project's name. The four oldest download links carried fixed names in the markup, so captions for every project arrived as hyperaudio.vtt and the operating system told them apart with a number. They use the shared naming helper now, as the document and media exports already did, set on a library change and again on the way out so a rename lands. An untitled project keeps the markup's name.

Fixes #636
Fixes #637

Gate: 122 unit; 366 e2e passed, 1 skipped (the known WebKit OPFS skip).

#636 is not reproducible headlessly: the same sequence in Chromium and WebKit paints one cue with or without the fix, which is what a compositing artefact under software rendering looks like. It was verified by hand in the browser that shows it.