feat: ap.visual-editor.background-controls filter (#649) - #652

Adds a shared JS filter that lets external packages contribute panels
to the background/appearance area of any block that opts into a
background support, without each package enumerating target blocks and
wrapping editor.BlockEdit itself.

The visual editor now owns the target-block decision via a single
editor.BlockEdit HOC that gates on supports.background and
supports.color.background, then renders filter-registered controls
sorted by priority (default 10, lower first) and deduped by id
(last-wins).

Downstream packages like liquid-glass can drop their per-package
withBlockEdit HOC and register a single filter callback instead.

Closes #649

- Register the BlockEdit HOC BEFORE responsive/state HOCs so it wraps
  innermost. context.attributes is now the breakpoint-merged /
  state-resolved view (not raw idle) and context.setAttributes routes
  writes through the responsive/state wrappers.
- Wire registerBackgroundControls into the site-editor bootstrap so
  the filter fires when editing templates, template parts, and
  patterns (previously dead in the site editor).
- Dedupe by id BEFORE priority sort so "last-wins mirroring
  @wordpress/hooks" tracks registration order, not sort order — a
  later addFilter overrides an earlier one at the same id regardless
  of relative priorities.
- Deep-clone context.blockSupports (structuredClone) so a filter
  callback can't corrupt @wordpress/blocks' registry via mutation.
  Freeze context.attributes and type both fields as Readonly<>.
- Wrap applyFilters in try/catch; log to console.error and return an
  empty list on throw so one bad third-party filter can't trip
  BlockCrashBoundary for every block in the session.
- Reject controls with a non-finite priority (NaN, strings, Infinity)
  at validation time — engine-dependent sort order eliminated.
- Reject empty-string id at validation time.
- Always return the Fragment shape from the HOC — a 0→1 controls
  transition no longer remounts BlockEdit and lose RichText cursor /
  drag / child hook state.
- Drop the dead `data-background-control` wrapper div.
- Move `ap.visual-editor.rendered-block` docs to the PHP filters
  section with correct description (it's a server-side PHP filter,
  not a save-time JS filter).
- Fix the docs example gate to check both `blockSupports.background`
  and `blockSupports.color.background` — the naive `!blockSupports.background`
  check would silently no-op on color-background blocks.

Overlaps with CodeRabbit review comments on PR #652 (priority
type-check + resolveBlockSupports live-reference leak).

New tests cover: registration-order-over-priority dedupe (both
directions), NaN/string/Infinity priority rejection, empty-id
rejection, thrown-callback swallowing, attributes-freeze semantics,
blockSupports deep-clone semantics, always-Fragment BlockEdit
stability across empty→non-empty controls transitions.

Test count: 20 → 27 in background-controls; full editor suite 1861 →
1868 passing (site-editor-app.test.tsx pre-existing failure remains,
now surfacing as a transitive `@wordpress/blocks` JSON-import error
during collection rather than the previous 17 runtime failures — the
file has not passed on release/1.x for this reason).

Refs #649

The new site-editor bootstrap import of `registerBackgroundControls`
pulls the background-controls barrel into the site-editor-app test's
module graph, which transitively loads `@wordpress/block-editor` and
its `block-compare` → `diff` subpath — the same unresolved-under-jsdom
chain the file already stubs `BlockEditorBoundary` and
`BlockLibrarySidebar` for.

Stub `../../background-controls` to a no-op registrar so the shell
test can boot again on CI. The HOC and filter helpers have their own
focused suites under `background-controls/__tests__/`.

Refs #649