A fresh full-stack re-review checked 15 PR heads independently. Cross-
checked all 9 remaining claims against the actual current tip:

- #2120 (id/selector key qualification, Hide All no-op/race, variable
  parity), #2121 (opacity-zero fallback), #2122 (GSAP preview sibling
  resolution, scrub-label), #2124/#2126 (negative metadata cache), and
  the keyboard-access half of #2121/#2186 were all already fixed by a
  later commit in this same stack (65954c3, PR #2225) — the reviewed
  heads predate it. Verified each in the current source rather than
  taking the isolated-head review at face value.

- #2186's "no ESC/right-click cancel during drag" was the one claim that
  held up: FlatSlider had keyboard arrow-key support but no way to abort
  an in-progress pointer drag. Escape now reverts to the pre-drag value
  and releases pointer capture; a right-click (contextmenu) during a drag
  does the same instead of committing whatever position the pointer last
  reached while the native context menu opens over the slider. Both go
  through commitDraft (not just a visual reset) since the drag's leading-
  edge commit in onPointerDown may already have applied an intermediate
  value that needs actually undoing, not just hiding.

propertyPanelFlatPrimitives.tsx crossed the 600-line file-size gate after
this change; extracted FlatSelectRow into its own file, matching the
FlatToggle/FlatMaskInsetRows precedent from earlier in this stack.

New regression tests for Escape-cancel and contextmenu-cancel. Full
studio suite still at the known pre-existing 55-failure baseline, zero
new regressions. Typecheck/oxlint/oxfmt clean.

## What

First PR in a 6-PR stack migrating Studio's right-panel property inspector from an always-expanded stacked-sections layout to a "flat" one-open-at-a-time accordion. This PR lays the foundation: the `STUDIO_FLAT_INSPECTOR_ENABLED` feature flag, the accordion primitives (`FlatRow`, `FlatSegmentedRow`, `FlatGroup`, `PinnedZoneDivider`), the flat identity header/footer, and the first migrated group — Text.

Stack: #2120 (this) → #2121 (Style) → #2122 (Layout+Motion) → #2123 (Media) → #2124 (Grade) → #2125 (Pinning + multi-field Text).

## Why

The legacy inspector renders every applicable section expanded at once, which gets unwieldy as an element accumulates properties across style/layout/motion/media/grade. The flat redesign shows one section at a time (plus pinned sections), matching a design handoff mock.

## How

- `FlatGroup` owns the one-open accordion state (`openGroupId`/`onToggleOpen`) and pin affordance (`onTogglePin`), styled per the design mock.
- `FlatTextSection` is the first migrated group and the reference implementation every later group's task followed for the `isOpen`/`onToggleOpen`/`onTogglePin`/`summary` wiring pattern.
- Includes a same-PR bugfix (found via live browser testing, not caught by any automated test): the Text `FlatGroup` was rendering unconditionally regardless of element type (empty for non-text elements), and the multi-field fallback doubled the "Text" heading. Fixed by gating on `isTextEditableSelection` and adding a `hideOwnHeading` prop to the legacy `TextSection` fallback.
- Entirely gated behind `STUDIO_FLAT_INSPECTOR_ENABLED` (default off) — the legacy panel is untouched and remains the default for all users.

## Test plan

- Every primitive and the Text group have dedicated Vitest suites using real DOM events (click/pointerdown) with exact assertions, not shallow snapshots.
- Manually verified in Studio via live browser testing against the design mock (this is what caught the bugfix above).
- Full monorepo test suite green; `oxlint`/`oxfmt` clean; this repo's `fallow` complexity/duplication gate passes.
- [x] Unit tests added/updated
- [x] Manual testing performed
- [ ] Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)

A fresh full-stack re-review checked 15 PR heads independently. Cross-
checked all 9 remaining claims against the actual current tip:

- #2120 (id/selector key qualification, Hide All no-op/race, variable
  parity), #2121 (opacity-zero fallback), #2122 (GSAP preview sibling
  resolution, scrub-label), #2124/#2126 (negative metadata cache), and
  the keyboard-access half of #2121/#2186 were all already fixed by a
  later commit in this same stack (65954c3, PR #2225) — the reviewed
  heads predate it. Verified each in the current source rather than
  taking the isolated-head review at face value.

- #2186's "no ESC/right-click cancel during drag" was the one claim that
  held up: FlatSlider had keyboard arrow-key support but no way to abort
  an in-progress pointer drag. Escape now reverts to the pre-drag value
  and releases pointer capture; a right-click (contextmenu) during a drag
  does the same instead of committing whatever position the pointer last
  reached while the native context menu opens over the slider. Both go
  through commitDraft (not just a visual reset) since the drag's leading-
  edge commit in onPointerDown may already have applied an intermediate
  value that needs actually undoing, not just hiding.

propertyPanelFlatPrimitives.tsx crossed the 600-line file-size gate after
this change; extracted FlatSelectRow into its own file, matching the
FlatToggle/FlatMaskInsetRows precedent from earlier in this stack.

New regression tests for Escape-cancel and contextmenu-cancel. Full
studio suite still at the known pre-existing 55-failure baseline, zero
new regressions. Typecheck/oxlint/oxfmt clean.

A fresh full-stack re-review checked 15 PR heads independently. Cross-
checked all 9 remaining claims against the actual current tip:

- #2120 (id/selector key qualification, Hide All no-op/race, variable
  parity), #2121 (opacity-zero fallback), #2122 (GSAP preview sibling
  resolution, scrub-label), #2124/#2126 (negative metadata cache), and
  the keyboard-access half of #2121/#2186 were all already fixed by a
  later commit in this same stack (65954c3, PR #2225) — the reviewed
  heads predate it. Verified each in the current source rather than
  taking the isolated-head review at face value.

- #2186's "no ESC/right-click cancel during drag" was the one claim that
  held up: FlatSlider had keyboard arrow-key support but no way to abort
  an in-progress pointer drag. Escape now reverts to the pre-drag value
  and releases pointer capture; a right-click (contextmenu) during a drag
  does the same instead of committing whatever position the pointer last
  reached while the native context menu opens over the slider. Both go
  through commitDraft (not just a visual reset) since the drag's leading-
  edge commit in onPointerDown may already have applied an intermediate
  value that needs actually undoing, not just hiding.

propertyPanelFlatPrimitives.tsx crossed the 600-line file-size gate after
this change; extracted FlatSelectRow into its own file, matching the
FlatToggle/FlatMaskInsetRows precedent from earlier in this stack.

New regression tests for Escape-cancel and contextmenu-cancel. Full
studio suite still at the known pre-existing 55-failure baseline, zero
new regressions. Typecheck/oxlint/oxfmt clean.