Studio pillar rail: one 240 px rail (uppercase heading with count and an add button, search, 32 px rows, group headers) replaces the four per-pillar w-52 rails #12448

Source

Epic #12446, decisions 1 and 3. Measured on objectui main a2225f91: DataPillar, AutomationsPillar and AccessPillar each render their own flex w-52 shrink-0 flex-col border-r rail with an 11 px muted heading, a search input and 13 px rows; InterfacesPillar renders a nav tree with two-line rows. Headings sit at different y offsets from the content column's first control, and the Data rail ends in a "Read-only" footer.

Target

One shared rail component (StudioRail.tsx) used by all four pillars:

• Width 240 px, bg-sidebar-style surface, right border, full height under the shell header.
• Header row, 44 px: an 11 px uppercase heading with 0.06 em tracking, an 18 px count pill, and at the right a 28 px add button (hidden when the package is read-only; the read-only card owns that rule).
• Search input, 32 px, with a leading search glyph; placeholder per pillar from the existing keys.
• Rows, 32 px, 6 px radius, 13 px text, 10 px side padding: a 16 px glyph slot, a truncating label, an optional trailing slot (a status dot, a kind tag, a count). Active row: primary-soft background, primary text, 500 weight. Hover: a muted wash.
• Group header row, 28 px, 6 px top margin: an 11 px uppercase label, a trailing chevron; child rows indent by 12 px. Groups collapse and remember their state per pillar in session memory (not persisted metadata).
• An empty-group row: 28 px, 12 px muted text ("No items in this package").
• Optional legend slot at the bottom (the Automations card uses it).

The Data rail's "Read-only" footer is removed here; the read-only card re-states that signal elsewhere. The Objects rail keeps the per-object icons from metadata; a data-testid per pillar rail (studio-rail-data, -automations, -interfaces, -access) is added for the pillar cards' tests.

File surface

New packages/app-shell/src/views/studio-design/StudioRail.tsx; StudioDesignSurface.tsx (the four rail blocks swap to the component; row content stays as each pillar renders it today). Rail search behavior (AutomationsPillar.railSearch-11794, navRailLabels-11862, accessRailLabels-11862, railLabelInherits-11196, railSeparatorBadge-11791, railUnboundReason-11281) is preserved, not redesigned, by this card.

Acceptance

1. All four pillars render the rail at 240 px with the heading, count, add button and search at identical y offsets (screenshot of the four side by side at 1280 px).
2. Row height 32 px and the active-row style verified by a test that mounts two pillars and compares computed classes.
3. The six rail tests above stay green unchanged.
4. Before/after screenshots, light and dark.

Constraints

• Tailwind utilities and theme tokens only; no inline style.
• Existing data-testids and i18n keys stay; new test ids listed above.
• Gates: pnpm --filter @object-ui/app-shell test, lint, build.
• Depended on by the Data, Automations, Interfaces and Access pillar cards; land it first.

Parent: #12446