fix: English aria-label fallbacks when locale keys are unresolved (#196)

[johnleider]

Closes #196.

Problem

Components emit user-facing strings (aria-label, aria-valuetext, live-region text) via locale.t(key). v0 ships no default message bundle, so when no locale is configured — the out-of-box default — t() returns the key verbatim. Screen readers then announce dotted keys like "Dialog dot close", "Pagination dot next" across ~16 components. That's a WCAG 4.1.2 (Name, Role, Value) defect in the default configuration.

Fix

Each call site coalesces to a concise English default when t() returns the key unchanged:

const close = locale.t('Dialog.close')
// aria-label:
close === 'Dialog.close' ? 'Close' : close

This is the per-component default approach already established in AvatarIndicator (translated === LABEL_KEY ? \+${count}` : translated) — not a new mechanism. It deliberately avoids the two previously-rejected approaches: it does **not** add a fallback parameter to t()`, and it does not ship a default message bundle. A consumer-supplied locale always overrides; the English default is only the out-of-box accessibility floor.

Interpolated keys (Carousel slide counts, Pagination page numbers, Rating value text, the Pagination/Carousel live regions) keep their interpolation in the fallback.

Scope

• ~16 components across Dialog, AlertDialog, Snackbar, NumberField, Button, Rating, Breadcrumbs, Pagination (incl. live status), Carousel (incl. live region)
• PHILOSOPHY.md §5.5 updated to document the fallback as the accessibility floor
• Breadcrumbs / NumberField / Pagination tests updated — they previously selected elements by, or asserted equality with, the raw key (which also violated the documented "assert toBeDefined(), not exact text" rule)

[pkg-pr-new]

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commit: d46013e