Proposal H, Step 2 PR 2a: real candidate/version picker in the display rail

[WilfordGrimley]

Description

Instrument parity, PR 2a of the Step 2 sequence for docs/proposals/proposal-h-unified-display-page.md
(behind NEXT_PUBLIC_UNIFIED_DISPLAY_ENABLED, unmerged Step 1 shell from PR #87).

The rail's "Choose Image" accordion section previously rendered a stub. This PR wires it up to the
real candidate/version-picker machinery, extracted out of GridSelectorModal.tsx so the classic
modal and the new rail share one implementation instead of forking it:

• useGridSelectorSearch.ts (new) — GridSelectorModal's entire filtering/sorting/search state
  machine, extracted verbatim into a hook (show renamed active).
• GridSelectorResults.tsx (new) — the filters-column + results-column body, extracted out of the
  modal. Takes a variant: "modal" | "embedded" — "modal" wraps each column in the existing
  OverflowCol (unchanged behavior); "embedded" uses a plain Col so the display rail's own
  scroll container isn't fighting a second, nested one.
• GridSelectorModal.tsx — refactored to consume both of the above. Its own behavior is unchanged
  and is proven so by its full existing Playwright suite (GridSelectorModal.spec.ts,
  GridSelectorModalAccessibility.spec.ts, GridSelectorModalMobile.spec.ts — 26 tests), plus
  CardSlot.spec.ts and DeckbuilderConfirmAffordance.spec.ts, all passing unmodified.
• DisplayPage.tsx — "Choose Image" now renders a ChooseImageSection that resolves the selected
  slot's candidates the same way CardSlot.tsx does and dispatches the same setSelectedImages
  action on selection, so the sheet updates immediately.
• CardResultSet.tsx — its result grid's column count (row-cols-*) is driven by viewport width,
  correct for the classic full-width modal but not for the rail's much narrower results column. Added
  a variant prop so "embedded" pins a fixed 2-column layout regardless of viewport size, instead of
  squeezing candidate cards down to a few px wide.

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate.
• I have manually tested my changes as follows:
  • Full DisplayPage.spec.ts suite (9 tests, incl. the embedded candidate grid's real selection flow and no-nested-scroll-region check).
  • Full GridSelectorModal/GridSelectorModalAccessibility/GridSelectorModalMobile/CardSlot/DeckbuilderConfirmAffordance suites (55 tests) to confirm zero behavior change to the classic modal.
  • npx tsc --noEmit clean.
  • npx jest — 345 tests passing.
  • NEXT_PUBLIC_UNIFIED_DISPLAY_ENABLED=true npx next build — clean production build with the flag on (the standing verification bar added after PR Fix production build: RailProps.cardDocumentsByIdentifier needs | undefined #90's build failure).
• I have updated any relevant documentation or created new documentation where appropriate.

Per the Step 2 instructions: zero changes to the classic editor's/modal's behavior (verified above);
"After 2f: STOP" — steps 3/4 (inline export, switchover) are not part of this or any remaining Step 2 PR.

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