feat(components): freeze tab widths during rapid closes like Chromium

[wibus-wee]

Summary

Implements Chromium's rapid-close mode (in_tab_close_ / override_available_width_for_tabs_) in AdaptiveTabStrip, so closing a tab keeps every survivor at its current width and the next close button lands under the cursor — matching Chrome's behavior during repeated closes.

• Pointer-armed freeze: a pointerdown inside the strip arms the gesture (≈ CloseTabSource::kFromMouse); keyboard/programmatic closes relayout normally.
• Chromium exit semantics: the freeze releases when the pointer leaves an expanded MouseWatcher region (40px below the strip, 60px toward the new-tab button), on a tab add, on a viewport shrink below the captured width (growing keeps it, min(override, real)), or when a single tab remains. Closing the last tab while frozen re-spreads survivors over the occupied width, keeping the new last tab's close button under the cursor.
• Role-based frozen widths: a survivor promoted to active takes the captured active width (size_delta = next_active_tab->width()), and widens in place without the removal slide so it cannot read as unfolding from the emptied slot.
• 200ms transitions: removals slide closed via margin-inline-start; pure inserts grow from zero (StartInsertTabAnimation); a same-commit remove+add morphs from the removed width (draft→session promotion); a lone tab in an empty strip renders directly.
• Render-phase freeze decision: the removal diff runs during render (React derived-state adjustment), so a removal commit's first painted frame already carries frozen widths. An earlier useLayoutEffect-driven freeze let the unfrozen allocation commit first and flashed survivors at the fresh width for a frame.
• Storybook Rapid Close story is now fully interactive (close/select/reorder/new-tab/restore against local state) and selects the same neighbour as production's getSessionTabFallback.

Test plan

• tests/session-tab-bar.test.tsx rapid-close suite — 24/24 pass: freeze, slop boundaries, consecutive closes, slide margins, trailing-close re-spread, single-survivor release, viewport grow/shrink, unarmed close, active-close promotion, substitution morph, empty-strip insert, and a MutationObserver regression asserting the active item never holds an unfrozen width on the removal commit
• tests/adaptive-tab-strip.test.ts — 7/7 pass
• Verified in Storybook (Sessions/SessionTabBar → Rapid Close): freeze, slop region, trailing-close re-spread, margin/width CSSTransitions via document.getAnimations(), and per-mutation style sequences showing no unfrozen width
• Exercised in a running desktop build

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