Replace the card-chooser selection pill with a shared boxel-ui SelectionMenu

[lukemelia]

The "N selected" bulk-selection trigger in the card chooser was a flat teal pill — a visual regression from the previous dropdown (CS-11325). This makes it a proper primary dropdown control and extracts the whole thing into a reusable boxel-ui component so other surfaces (e.g. the workspace CardsGrid — CS-11366) can adopt it.

Folds in the work formerly split across #5100 + #5101 (now closed): the extraction reworks the same trigger, so they're reviewed together as one net change.

Decision (CS-11325)

BoxelSelect is the wrong primitive here — it's a value-picker, but this is an action menu (Select All / Deselect All). So this keeps BoxelDropdown + Menu and fixes the trigger, rather than reaching for a value-picker or polishing the flat pill.

What's here

• New generic, content-agnostic SelectionMenu in boxel-ui. It owns the trigger — a primary BoxelButton with a selection checkmark, the count, and a caret that flips on open — and the dropdown shell. The consumer supplies the menu contents via @items, so Select All / Deselect All and the inert count header stay in the app (built in SearchResultHeader); app-specific selection semantics don't leak into the design system. @label sets the trigger's accessible name.
• SelectionCheckmark artwork moved into boxel-ui and reused in CardHeader, removing a byte-identical inline SVG (advances CS-11333).
• Focus + interaction polish: keyboard focus shows a ring just outside the trigger without darkening the fill (CS-11328); the trigger holds its position when the menu opens (wrapped so basic-dropdown's content-origin doesn't add a flex gap).

Scope / surface

Touches boxel-ui's public surface (new SelectionMenu / SelectionCheckmark exports) and host. No behavior change for the card chooser: same markup, data-test-* hooks, aria-label, and menu items. boxel-ui + host both pass ember-tsc (0 errors), eslint, and prettier; verified in-browser across resting / hover / open / keyboard-focus.

Stacked on #5099 (CS-11313); auto-retargets to main when that merges. Follow-up: CS-11366 (bring SelectionMenu to the workspace CardsGrid).

🤖 Generated with Claude Code

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

• Boxel UI staging preview

Host Test Results

    1 files  ±0      1 suites  ±0   1h 50m 19s ⏱️ - 2m 42s
2 931 tests ±0  2 916 ✅ ±0  15 💤 ±0  0 ❌ ±0 
2 950 runs  ±0  2 935 ✅ ±0  15 💤 ±0  0 ❌ ±0 

Results for commit 2746dc8. ± Comparison against earlier commit ed0f9c7.

Realm Server Test Results

    1 files  ±0      1 suites  ±0   10m 14s ⏱️ -25s
1 549 tests ±0  1 548 ✅ ±0  1 💤 ±0  0 ❌ ±0 
1 640 runs  ±0  1 639 ✅ ±0  1 💤 ±0  0 ❌ ±0 

Results for commit 2746dc8. ± Comparison against earlier commit ed0f9c7.

[Copilot]

Pull request overview

Updates the bulk-selection dropdown trigger in the card-search “search result header” to read as a primary dropdown button (highlight fill + foreground, hover/open state deepening, external focus ring, and caret rotation) while keeping the existing BoxelDropdown + Menu behavior unchanged.

Changes:

• Restyles .selection-dropdown-trigger as a primary/highlight button (tokens, sizing, spacing, transitions).
• Separates hover/open fill deepening from keyboard focus (outline ring + offset, no focus-darken).
• Rotates the dropdown caret when [aria-expanded='true'] to match standard dropdown affordance.

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