Resolve the pilot package drift the gate, the lint rules and decision 0004 each found - #1175

… findings

Three separate checks each found real drift in this package, none of them
looking for it: the state-idiom review behind decision 0004, the StyleX lint
rules the package's own patterns produced, and the first run of the visual and
accessibility gate. All three were deferred on the same prerequisite — a working
visual gate for this package — which the gate now supplies, so they are settled
together rather than three times over.

Raw colours become semantic tokens. `on-primary` is the foreground for anything
drawn on a `primary` background; `shadow-raised` is the popover elevation and
takes its default from the `shadows.lg` scale step rather than restating the
rgba stack. Both defaults reference a named scale value, so the palette choice
stays in one reviewable file.

`primary` moves from `blue500` to `blue600`, and that is a legibility fact
rather than a preference. The accessibility gate failed ten of this package's
thirty-nine stories on exactly one thing: white body text on the selected
segment measured 3.76:1 where AA requires 4.5:1. `blue600` measures 5.25:1.
Renaming the literal to `on-primary` would have preserved the violation behind a
nicer name. `accent` follows `primary` so the brand blue stays one colour.

The thirteen deprecated top-level pseudo-class sites move into condition
objects. This is the change the deferral was actually about, because nesting a
pseudo-class changes which condition wins, so every site was checked for a
competing state on the same property rather than translated mechanically. Eleven
of the thirteen set a property nothing else touches and carry no precedence risk
at all. The two that do compete — the segmented control's and the list option's
background under hover versus selection — are resolved by application order
instead: hover lives in the base style, selection is a later `stylex.props`
argument, and it restates the hover value because a later unconditional
`backgroundColor` does not replace an earlier `backgroundColor` under a `:hover`
key. That is the R06 shape, and it means the outcome no longer depends on
attribute conditions outranking pseudo-class ones.

Focus moves from `:focus` to the accessible-component library's own
focus-visible state where the element is one of its components, and to the
native `:focus-visible` on the one plain `<input>` that is not. A pointer click
no longer paints a keyboard focus ring. Note what this is NOT: the ring is still
drawn with `boxShadow` rather than converted to `outline`. The partitioning
invariant already holds here — nothing else on these elements sets `outline*`,
and the lint rule confirms it — and no story exercises a focused element, so
repainting the ring would be a visual change the gate structurally cannot
adjudicate. Recorded as follow-up rather than done blind.

State resolution stops re-deriving what the component already knows. The
checkbox derived its box style from its own `value` prop; selection lives on the
CheckboxButton, which is an ancestor of the box, so the box cannot read it as
one of its own conditions and React Aria's render prop is the sanctioned
mechanism for that case. The segmented control and the list options branched
between two mutually exclusive style objects, hover rule included; they now
apply one additive override in argument order.

The accessibility gate's eleventh failure was structural, not colour. React
Aria's `Header` renders `<header>`, and a `<header>` outside sectioning content
is a `banner` landmark, so two nested field groups produced two banners and
tripped `landmark-no-duplicate-banner` and `landmark-unique`. The label is now a
plain element and the group carries the accessible name it was missing.

The seventeen per-line `oxlint-disable` suppressions are gone; the package is
gated by all eight rules again.