sdk: release 0.2.21

[foxnne]

What changes

sdk_version goes from 0.2.20 to 0.2.21. Merging tags sdk-v0.2.21, publishes its tarball,
and asks the store plugins to repin.

Why now. The plugin boundary's fingerprint moved since sdk-v0.2.20 (0x7b455e7a… →
0x1bf417f6…, by #331). Store plugins built against 0.2.20 do not load in a fizzy built from
main, and fizzyed.it/app is built from main. The chat plugin (fizzyedit/chat) also needs
#331's Host.documentTitleChanged, and builds against a local fork until this is out.

SDK impact

• None
• Core-only or additive: reaches plugins at the next SDK release
• Fingerprint moved: recorded in sdk/src/version.zig, sdk_version untouched, PR labelled sdk
• SDK release: bumps sdk_version, lists the sdk PRs since the last sdk-v* tag

The sdk PRs since sdk-v0.2.20:

• sdk: a surface's title can change, and a document's tab follows its owner #331: a surface's title can change (Host.setSurfaceTitle, the Host keeping its own copy),
  and a document's tab follows its owner (Host.documentTitleChanged). documentTitle's slice
  now need only last the call.

Also reaching plugins (core/, sdk/replay):

• core: floats' screens are dvui's (dvui.screensSet) #325: floats' screens are dvui's (core/screens.zig).
• backend: viewports.Picture, fizzy.addDvui, the pointer over OS windows #326: replay.overlay draws through the backend's viewport picture.
• examples: replay-app moved to fizzyedit/example-app #301: sdk/replay_module.zig follows replay-app's move to fizzyedit/example-app.

Verified

• macOS:
  • zig build and test-sdk-version on this head.
  • scripts/pack-sdk.sh packs fizzy-sdk-v0.2.21.tar.gz.
  • Every store plugin (fresh pulls of pixi, zig, drive, atlas and ghostty main) builds against
    this tree's sdk/ with zig build --fork=<sdk> (31/31 and 28/28 steps) into a throwaway
    profile. The fork matched, and the built plugin reports pinned fizzy SDK: 0.2.21. The
    fingerprint moved, so each repin PR is a real rebuild, but none should open as a draft for a
    broken build. The agent and chat plugins build too (22/22, 19/19).
• Windows:
• Linux: CI.
• Web:

Follow-ups

• Each repin PR merged and its plugin tagged (the plugin repos' tags stay the maintainer's).
• fizzyedit/agent and fizzyedit/chat repin to the sdk-v0.2.21 tarball.

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