feat(plugin-selection): add programmatic setSelection() to apply/restore selections

[simonmysun]

Summary

@embedpdf/plugin-selection could only clear() a selection — there was no way
to set or restore one programmatically. This adds setSelection() to the
public capability so consumers can persist a selection and reapply it (after
reload, tab switch, etc.) or drive selection from code (e.g. search results,
"jump to citation").

The PR and the example is majorly made by vibe coding with human reviewing.

API

Added to both SelectionCapability and the document-scoped SelectionScope:

setSelection(range: SelectionRangeX | null, documentId?: string): PdfTask<void>;

SelectionRangeX is the exact { start, end } glyph-pointer shape already
emitted by onSelectionChange, so a saved value round-trips back in:

// save
let saved: SelectionRangeX | null = null;
selection.onSelectionChange((e) => { saved = e.selection; });

// restore (task resolves once highlight rects are computed)
selection.setSelection(saved).wait(() => {/* applied */}, ignore);

// clear
selection.setSelection(null);

Scope / notes

• Only the synthetic, glyph-range text selection has restorable state; the
  marquee (rectangular) feature is event-only and unaffected.
• Consistent with the existing clear(), this does not touch the native
  browser selection (apps suppress that via user-select: none, as the official
  examples do).

Changes

• packages/plugin-selection/src/lib/selection-plugin.ts — applySelection
  (wired to setSelection on capability + scope), validateRange,
  clampRangeToGeometry, normalizeRange.
• packages/plugin-selection/src/lib/types.ts — setSelection on
  SelectionCapability and SelectionScope with JSDoc.

Testing

Verified against the built dist with a headless registry + mock engine, and
with runnable React / Vue / Svelte / Preact demo apps temporarily deployed
at https://makelove.expert/tmp/sel-demo-dist/:

• apply a selection (single page + cross-page) and confirm highlight rects + onSelectionChange
• save → clear → restore reproduces identical rects
• reversed range normalizes; over-large index clamps; null clears
• invalid inputs (out-of-bounds page, negative/non-integer index, malformed object) are rejected without touching the engine
• switching to a disjoint selection clears stale highlights on the previously selected page

No public type removals or breaking changes (additive only).

[vercel]

@simonmysun is attempting to deploy a commit to the OpenBook Team on Vercel.

A member of the Team first needs to authorize it.