fix(webkit): [ENG-47063] flip, then shrink and scroll floating panels instead of clipping them

[isaque-bock-azion]

Summary

• usePlacement is now size-aware: flip first, and when no side fits, cap the panel to the free space (maxHeight/maxWidth in panelStyle) so it scrolls internally instead of overlapping its trigger or clipping at the viewport edge. The boundary is the nearest scrolling ancestor of the trigger intersected with the viewport, not the window.
• One positioning implementation remains: select-content and multi-select-content migrate onto usePlacement (the getFixedFrame Storybook-zoom correction moved into the composable, plus a matchTriggerWidth option), and the NavigationMenu positioner becomes a thin wrapper over the exported pure computePlacement / getClippingBoundary, keeping only its arrow, rAF coalescing and settled-size handling. Its --available-height / --available-width now carry the real cap instead of a hardcoded 100vh / 100vw, and the viewport scrolls while data-constrained.
• Popover, Dropdown, Calendar, Table filter/column selector and DocTooltip panels gain overflow-y-auto / min-h-0 so the computed cap produces scroll; new browser-mode tests cover flip, shrink and the scroll-container boundary for the composable, Select, MultiSelect, Popover and NavigationMenu.

How to test

1. pnpm webkit:test — 109 files pass, including the new src/composables/use-placement/use-placement.test.ts and the flip/shrink cases in select.test.ts, multi-select.test.ts, popover.test.ts and navigation-menu.test.ts.
2. pnpm storybook:dev, open Components/Inputs/Select → LongList, shrink the browser window to ~420px tall and open the select: the list is capped to the space below the trigger, ends 8px above the viewport edge and scrolls inside the panel.
3. In the same story, move the trigger near the bottom of the preview (e.g. add style="position:fixed;bottom:8px" to #storybook-root in DevTools) and open it: the list opens above the trigger (--popup-origin: bottom left), never overlapping it.
4. Set the Storybook zoom to 125% and open the select: the panel stays aligned to the trigger's left edge and width.
5. Open Components/Navigation/NavigationMenu → Default, shrink the window to ~260px tall and hover "Soluções": the popup is capped 8px above the viewport edge, the positioner carries data-constrained, the panel scrolls, and the arrow stays centred on the trigger. Hover "Produtos": the box morphs to the new width and keeps the cap.
6. pnpm --filter webkit size — every budgeted entry is within its (raised, see Notes) budget.

Notes

• Related issue: ENG-47063 — all four work items and every acceptance criterion are in this PR.
• Behaviour change (not an API break): an explicit placement that fits on neither side now resolves to the roomier side and is capped, instead of being pinned to the top edge over its trigger. panelStyle / resolvedPlacement are now Readonly<Ref>; all 8 call sites keep working unchanged. The NavigationMenu positioner's public props are unchanged; it gains a data-constrained attribute.
• Bundle budgets raised by 0.5 KB each for calendar-root, chip, doc-prompt, doc-tooltip, doc-page-header: the composable grew ~0.9 KB gzip (size cap, scroll-ancestor boundary, frame correction, ResizeObserver), inherited by every overlay consumer. The old code lacked all three behaviours the ticket requires, so the growth is the feature, not slack.
• Select's --popup-origin changes from top to top left, so the open-state visual baselines for Select/MultiSelect may need regenerating.
• The 4 "ResizeObserver loop completed with undelivered notifications" lines in the full Vitest run are pre-existing on main (same count, verified against 444e357).
• Spotted, out of scope: multi-select-content lacks the trigger-contains guard in onDocumentPointerDown that Select got in ENG-46739; .specs/select.md is still status: draft and both select specs describe a Popover-API implementation that never shipped; NavigationMenuPositioner's sticky prop is declared but never read.