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refactor(a11y): project composite ARIA onto the native editors (#2327)

* refactor(a11y): project composite ARIA onto the native editors

Shadow DOM breaks two things ARIA depends on: the identity of the
focused element (delegated focus lands on the native editor inside the
input's shadow root, not the host holding the semantics) and IDREF
resolution (a reference cannot cross a shadow boundary). Select, combo
and the date pickers each worked around this ad hoc, and select's
combobox semantics were published on an element assistive technology
never reports on.

Replace the per-component workarounds with shared infrastructure:

- ARIA projection controller pair (`addAriaProjector`/`addAriaTarget`):
  composite hosts declaratively project role, popup state and relations
  onto the native editor of their inner input component. Relations
  travel as ARIA element references, which resolve across shadow
  boundaries, and the resolved bindings are applied with a single
  `ariaBindings()` element-expression directive. References are
  re-asserted every render — the browser drops them when assigned to a
  detached element (lit's first render commits inside the template
  fragment).
- External label forwarding is folded into the same channel, replacing
  three copy-pasted implementations; textarea previously did not
  forward host labels at all.
- `internalsOf()` registry lookup so internal code reaches a
  component's internals controller without `@hidden @internal` public
  members; `_ariaProperties`, `_labelElements` and `_labelTarget` are
  deleted.
- `reflectRole` option on the internals controller centralizes the
  "axe only reads content attributes" workaround, removing the seven
  manual `this.role` assignments.
- Both date pickers now project `describedBy` onto their editors,
  fixing a dead helper-text association. Combo's wrapper-level
  attributes move onto the native input (`aria-owns` dropped,
  `aria-haspopup="listbox"` added) and its clear icon g... (continued)

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