…idden default

Per WAI-ARIA 1.2 §6.6 + aria-hidden value table, a missing or empty-string
aria-hidden resolves to default `undefined` — NOT `true`. Valueless
`<button aria-hidden>` and empty `<button aria-hidden="">` are therefore
NOT spec-hidden; they do not create a focus-trap anti-pattern and the
rule should not flag them.

The prior behavior inherited jsx-a11y's convention (jsx-ast-utils
coerces valueless JSX attrs to boolean true) and vue-a11y's
"anything-not-literal-false" shortcut. Both are peer-plugin conventions,
not normative ARIA interpretations. Matching ember-cli#2717's spec-first
resolution.

Also corrects the rule-doc comment: the claim attributed to WAI-ARIA 1.2
("Authors SHOULD NOT use aria-hidden='true' on any element that has
focus or may receive focus") is not in the WAI-ARIA spec. The spec only
says authors MAY "with caution" use aria-hidden. The rule's concern
(keyboard trap) comes from community/axe guidance, which this comment
now accurately attributes.

Net: flagged values are now `aria-hidden="true"` (ASCII case-insensitive),
`aria-hidden={{true}}`, and `aria-hidden={{"true"}}`. Valueless, empty,
`false`, and `{{false}}` are all accepted.

…ec + correct peer-plugin claims

Two corrections to the previous revision:

1. Valueless / empty-string `aria-hidden` is no longer treated as a
   non-interactive escape hatch. Per WAI-ARIA 1.2 §6.6 + aria-hidden
   value table, a missing or empty-string value resolves to the default
   `undefined` — NOT `true`. Only an explicit `aria-hidden="true"`
   (ASCII case-insensitive) or mustache-literal `{{true}}` opts out.
   This matches ember-cli#2717 / #19's spec-first resolution.

2. Code comment corrections. jsx-a11y's util is named
   `isPresentationRole`, not `hasPresentationRole`. The comment also
   claimed jsx-a11y's `isPresentationRole` does "first token of a
   space-separated role list" — it does not (jsx-a11y does plain
   `presentationRoles.has(rawValue)`, no trim/lowercase/split). Our
   first-token behavior is a deliberate superset, not parity.

Moved `<div aria-hidden onclick>` and `<div aria-hidden="" onclick>` from
the valid section to invalid. Added `<div aria-hidden="TRUE">` as
additional valid coverage for the case-insensitive path.

…ML boolean semantics

Per HTML Living Standard on boolean attributes, the presence of `autofocus`
indicates TRUE regardless of value — `autofocus="false"` and
`autofocus="autofocus"` are equally truthy. jsx-a11y's `no-autofocus`
treats the literal string `"false"` as an opt-out (via `getPropValue`),
but that's a peer-plugin convention that diverges from HTML semantics;
vue-a11y and lit-a11y are presence-based, consistent with the spec.

Narrow opt-out to the only case that is spec-consistent:
- `autofocus={{false}}` in angle-bracket syntax — renders no attribute.
- `{{input autofocus=false}}` in mustache hash-pair syntax — no attribute.

Revert peer-parity opt-outs for `autofocus="false"`, `autofocus={{"false"}}`,
and `{{input autofocus="false"}}` — these are now flagged per HTML spec
semantics. Moved from valid → invalid in the test suite.

Dialog exemption unchanged — keeps MDN-backed behavior for autofocus on
and within <dialog>.

Follows the spec-first direction established in ember-cli#2717 (aria-hidden),
#19, #33.

…I-ARIA spec

Per WAI-ARIA 1.2 §6.6 + aria-hidden value table, a missing or empty-string
aria-hidden resolves to the default `undefined` — NOT `true`. So
<span aria-hidden>X</span> as a child of <a href="/x"> does NOT hide the
span; its content still contributes to the anchor's accessible name.

The prior behavior inherited jsx-a11y's JSX-coercion convention and
vue-a11y's "anything-not-literal-false" shortcut. Both are peer-plugin
conventions that diverge from normative ARIA. Matches the spec-first
resolution of ember-cli#2717, #19, and #33.

Moved valueless / empty aria-hidden cases from invalid → valid. Kept the
explicit aria-hidden="true" and {{true}} cases as invalid.