docs(sdk-playground): accurate README with stage coverage and CanResult

[vanceingalls]

What

Rewrites packages/sdk-playground/README.md to accurately document what the playground does, what SDK stages it exercises, and what's still unimplemented.

Why

The original README had several inaccuracies that would mislead reviewers:

• Wrong ops table row — moveElement listed comp.moveElement(id, { parent, index }) but that method/signature doesn't exist. Drag-to-reposition uses comp.setStyle(id, { left, top }) internally. No explicit moveElement button in the ops panel.
• Outdated can() docs — described as returning a plain boolean; it now returns CanResult {ok: boolean, code?: string, message?: string, hint?: string} (landed in PR feat(sdk): can() returns CanResult; T4 dispatch-boundary tests #1426).
• Missing stage map — no explanation of which SDK stages the playground actually exercises, making it hard for reviewers to assess coverage.
• Keyframe ops not called out — addGsapKeyframe, setGsapKeyframe, removeGsapKeyframe are implemented in the SDK but have no UI in the playground; the old README omitted this gap.

Changes

• Added "What this demonstrates" table mapping playground features → SDK stages (3a session API, 3b GSAP engine, 4 partial: can/getOverrides/selection/find, 5 partial: FsAdapter/FileAdapter/PlaygroundPreview)
• Fixed ops panel table: removed incorrect moveElement row; added combined setGsapTween / removeGsapTween row; fixed addLabel/removeLabel dispatch syntax
• Updated can() docs to show CanResult shape with example
• Added Event log section (patch/undo/redo/selectionchange/persist:error all logged with full payload — useful for verifying RFC 6902 patch shape)
• Moved keyframe ops to Planned / not yet wired with note that SDK impls exist

Test plan

• README renders correctly on GitHub (tables, code spans)
• All ops panel table rows correspond to actual buttons in the playground (bun run --cwd packages/sdk-playground dev → Ops tab)
• "Planned" section accurately reflects what has no UI button

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(sdk): stage 6 — sub-composition scoped ids (F9) #1434
• feat(sdk): stage 5 — export adapter factories from package root #1432
• docs(sdk-playground): accurate README with stage coverage and CanResult #1429 👈 (View in Graphite)
• feat(sdk): stage 4 — canUndo/canRedo, removeElement GSAP cascade, override-set cleanup #1431
• feat(sdk): can() returns CanResult; T4 dispatch-boundary tests #1426
• fix(sdk): fs adapter flush() tracks in-flight writes; add to T13 contract suite #1425
• feat(sdk): file-backed fs adapter + setTiming GSAP-script sync; add sdk-playground #1423
• refactor(core): swap studio-api read path from recast to acorn parser (T6e) #1392
• feat(core): parse-parity suite for acorn parser (T6d) #1370 : 1 other dependent PR (#1379 )
• feat(core): acorn GSAP write path — magic-string offset-splice (T6c) #1369
• feat(core): acorn GSAP read path with T6b differential corpus tests #1368
• main

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