sdk: release 0.2.20

[foxnne]

What changes

sdk_version goes from 0.2.19 to 0.2.20. Merging tags sdk-v0.2.20, publishes its tarball,
and asks the store plugins to repin.

Why now. The plugin boundary's fingerprint has not moved since sdk-v0.2.19
(0x7b455e7a… both), so plugins built against 0.2.19 still load. But core/ and sdk/replay
changes merged since then reach plugins only through a release, and the agent plugin
(fizzyedit/agent, private until it can pin a released SDK) needs one of them: #295's
profiler on request.

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-v0.2.19: none; the fingerprint is unchanged.

What reaches plugins (core/, sdk/):

• core: the profiler records on request and reports as ZON #295: the profiler records on request (core.profile.want) and reports as ZON
  (core.profile.report). Profiler grew a field (abi 3 → 4), so a plugin records its own
  sections into the host's profiler again only once rebuilt against this release.
• keymap: a tape's key runs a command a macOS menu owns #310: replay.Input.synthetic, which tells a tape's key from the OS's.
• replay: every bundled demo plays and seeks exactly in test-integration #293: the replay Player plays and seeks every bundled demo exactly.
• automation: a run of the real app that plays a tape to a verdict #313: core.dialogs changes for a run of the real app that plays a tape to a verdict.
• backend: fizzy's native backend is a package of its own #312: core/gfx's programs and the liquid-glass shader, now that the native backend is a
  package of its own.
• ci: measure a plugin's rebuild loop on Linux, per backend and incremental #292: sdk/plugin_sdk.zig honours FIZZY_PLUGIN_USE_LLVM, for measuring a plugin's rebuild
  loop per code generator.

Verified

• macOS:
  • zig build and test-sdk-version on this head.
  • scripts/pack-sdk.sh packs fizzy-sdk-v0.2.20.tar.gz.
  • Every store plugin (fresh clones 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), installing
    into a throwaway profile. The fork matched, and the built plugin reports
    pinned fizzy SDK: 0.2.20. With the fingerprint unchanged, the repins should open as plain
    version bumps.
  • The agent plugin (path dependency on main's sdk/) builds, 21/21 steps.
• Windows:
• Linux: CI.
• Web:

Follow-ups

• Each repin PR merged and its plugin tagged (the plugin repos' tags stay the maintainer's).
• fizzyedit/agent repins from its local path to the sdk-v0.2.20 tarball.
• fizzyedit/example-plugin repins once PLUGIN_DISPATCH_TOKEN can reach it (Example repos that move with fizzy, and a dvui app on fizzy's backend #281, still open).

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