[miguel-heygen]

What

Release v0.9.21: version bump, releases/v0.9.21.md, and the docs changelog. Merging publishes to npm.

Why

Ships cloud uploads that send a big file in parts, so a cut connection resends one part (#7375), a re-encode that limits a large file to fit the cloud's size limit (#7376), and media given as a full path, Windows paths included, going into cloud exports (#7381). It also keeps HDR clips at the same brightness (#7349), plans a long film's chunks once instead of twice (#7382), limits checks to the compositions a project uses (#7369), and puts Safari's picture back on the cut (#7383). It carries the other changes on main since v0.9.20.

No visible change

Version bump and changelog only. Each change was captured in its own PR.

Test plan

• bun run release:prepare 0.9.21 --no-tag ran on current main (a fresh clone on a Linux box): it wrote the version bump. It did not write the notes, so bun run changelog:draft 0.9.21 --write wrote them and the summary sentence is by hand.
• One commit on top of current main, signed (the Linux box cannot sign, so the commit was made from the same patch on a signing machine). No tag was created or pushed.
• oxfmt --check on releases/v0.9.21.md and docs/changelog.mdx.
• CI on this PR, including the packed package size budgets.
• Desktop walked with this PR's packed packages: not run by me.

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[heygengenesis]

Approve at 0271d8ce. No blockers.

• Release-only diff: 21 manifests go from 0.9.20 to 0.9.21 (16 packages, 5 plugin/extension files). No 0.9.20 is left at the head. It also adds releases/v0.9.21.md and a docs/changelog.mdx entry, and the two have the same content.
• The base is current main (841e90ffe). The notes list all 19 commits in v0.9.20..main: 3 features, 13 fixes, 1 perf, 2 internal. Every SHA and PR link matches its commit. None is missing and none is listed twice.
• This isn't a fix PR, so it doesn't need a fails without the fix: line.
• CI at this head: CI, Docs, CodeQL, Lambda arm64 image, regression, preview-regression, Windows render verification, Player perf, Player Safari perf and PR captures all passed. Comments was skipped.

Nit (not blocking): the drafter keeps commit-subject casing, so the notes say "Hlg … sei … hlg's". "HLG … SEI … HLG's" would read better if you touch the notes again.