fix(sdk): bridge preview selection into session state

[kiyeonjeon21]

Summary

• Subscribe SDK sessions to PreviewAdapter selection events.
• Mirror preview-selected ids into getSelection() / selection().
• Emit selectionchange from the session and clean up the preview listener on dispose().

Why

PreviewAdapter.select() is documented as firing selectionchange on the session, and the SDK React embed example reads selection through comp.on("selectionchange") and comp.getSelection().

Before this change, openComposition(..., { preview }) stored the preview adapter but never subscribed to its selection events, so preview-driven selection was not reflected in the SDK session state.

Testing

• bun run --cwd packages/sdk test
• bun run --cwd packages/sdk typecheck
• bunx oxfmt --check packages/sdk/src/session.ts packages/sdk/src/session.test.ts
• bunx oxlint packages/sdk/src/session.ts packages/sdk/src/session.test.ts
• git diff --check

[vanceingalls]

Clean fix — the contract was documented in PreviewAdapter.on("selection") but never wired up in the session constructor. Implementation is correct: updateSelection mirrors the resolved selection without re-interpreting additive (that's the adapter's job before emitting), dispose cleanup is idempotent, and the three tests cover mirror, proxy dispatch, and leak prevention. Good catch, thank you.