feat(sdk): stage 7 step 2 — setSelection API (#1442)

* feat(sdk): stage 7 step 2 — setSelection API

Adds setSelection(ids: string[]) to Composition interface and CompositionImpl.
Fires selectionchange; does not touch undo stack or patch stream.
11 contract tests: get/set/clear, event firing, copy semantics, no undo/patch side-effects.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(sdk): guard setSelection against same-id no-ops

Skip event dispatch when ids are identical (same length, same order)
to prevent double-firing selectionchange from callers that call
setSelection with the same list. Two new tests (RED→GREEN verified).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(sdk): de-duplicate ids in setSelection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-authored-by: Miguel Ángel <miguel07alm@protonmail.com>

* fix(sdk): document PreviewAdapter.on("selection") as stage 8 prep

Reviewer noted it is dead surface in this stack — no caller uses it.
Add comment explaining it is wired up in stage 8 when the preview host
pushes selection events up to the SDK session.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-authored-by: Miguel Ángel <miguel07alm@protonmail.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Miguel Ángel <miguel07alm@protonmail.com>

packages/sdk/src/adapters/types.ts

@@ -70,5 +70,7 @@ export interface PreviewAdapter {
   /** Set preview selection; fires selectionchange on the session */
   select(ids: string[], opts?: { additive?: boolean }): void;
 
+  // Stage 8 prep: fired when the preview host changes selection (e.g. user clicks an element).
+  // Not wired up in stage 7 — callers listen to the session's own selectionchange event instead.
   on(event: "selection", handler: (ids: string[]) => void): () => void;
 }