fix(tabs): move the indicator when the text direction changes

[ndlabdev]

Summary

Tabs positions its sliding indicator from the active trigger's offset, and recomputed it only when value, orientation or items changed, or when the list resized. Flipping the text direction at runtime, for example a language switch that turns the page from ltr to rtl, mirrors the triggers without any of those, so the indicator kept its old offset and ended up under a different tab. The active label still rendered in the on-indicator color over the plain background, which made it look unreadable.

The indicator is now recomputed whenever a dir attribute changes in the document.

Closes #216

Type of change

• 🐛 Bug fix
• ✨ New feature / component
• 📖 Documentation
• ♻️ Refactor / chore
• ⚠️ Breaking change

Changes

• Tabs observes dir attribute changes (attributeFilter: ['dir'], subtree: true, from the document element) and reuses the existing rAF-debounced update. Watching the whole document rather than the current [dir] ancestors also covers a dir added to an ancestor later.
• The observer is disconnected on destroy, together with the pending animation frame.
• Small tidy in the same block: querySelector<HTMLElement> instead of a cast, and one cleanup instead of two.
• One regression test in the existing spec, without fixture files: with the first tab active, flipping the container to rtl and back keeps the indicator's left and width on the active trigger. It fails on dev (indicator at 4px while the trigger moved to 852px).
• CHANGELOG.md entry under [Unreleased].

Checklist

• Linked the related issue (Closes #…)
• pnpm check passes (0 errors, 0 warnings)
• pnpm lint passes
• pnpm test passes
• Added or updated tests for the change
• Updated CHANGELOG.md under [Unreleased]
• Followed component conventions (no comments outside *.types.ts, Material 3 design tokens)

Screenshots / notes

Verified in Chromium against dev, with the first tab active and a three-tab list:

• dir toggled on a wrapper element (the snippet from the issue): the active trigger moves from 4px to 459px. On dev the indicator stays at 4px; with the fix it follows to 459px and back.
• dir toggled on the html element, as a language switch would: same result.

Known limit: a direction set only through CSS, for example a class that applies direction: rtl, is not detected, since there is no attribute change to observe. The issue notes the same limit.

NavigationMenu measures its highlight bar the same way, but it is not affected: its root carries dir="ltr", so a wrapper flipping to rtl does not mirror it.

Only verified in Chromium.