feat(studio): flat inspector — Style group

[vanceingalls]

What

Second PR in a 6-PR stack for the flat inspector redesign (see #2120 for the foundation and full stack list). Adds the FlatSlider/FlatSelectRow primitives and the flat Style group (Fill, Stroke, Radius, Shadow, Blend, Layer blur, Backdrop, Overflow, Mask, Opacity), wired into the one-open accordion introduced in #2120.

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

Why

Style is the second-largest legacy section after Text and establishes the accordion's generalization from one group (Text) to N groups — every later group in this stack reuses the openGroupId/pinnedGroupIds pattern this PR generalizes.

How

• FlatSlider and FlatSelectRow are new shared primitives (drag-to-scrub numeric input, dropdown with tier-aware value coloring) used by every group in the rest of this stack.
• FlatStyleSection composes ten rows across Fill/Stroke/Radius/Shadow/Blend/Layer-blur/Backdrop/Overflow/Mask/Opacity, each built and reviewed as its own task.
• PropertyPanelFlat.tsx's accordion state generalized from a single hardcoded Text group to a repeatable per-group pattern (openGroupId === id || pinnedGroupIds.includes(id)).
• Still behind STUDIO_FLAT_INSPECTOR_ENABLED (default off).

Test plan

• Every new row/primitive has a real DOM-event test (click/pointerdown with exact assertions) — this convention, established here, was enforced task-by-task for the rest of the stack.
• Full monorepo suite green; oxlint/oxfmt/fallow clean.
• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(studio): Style/Grade/Text parity gaps found in legacy-retirement audit #2127 : 3 dependent PRs (#2128 , #2129 , #2134 )
• fix(studio): retire legacy Style and Grade sections from flat inspector #2126
• feat(studio): flat inspector — persisted pinning + multi-field Text #2125
• feat(studio): flat inspector — Grade (color grading) group #2124
• feat(studio): flat inspector — Media group #2123
• feat(studio): flat inspector — Layout + Motion groups #2122
• feat(studio): flat inspector — Style group #2121 👈 (View in Graphite)
• feat(studio): flat inspector foundation + Text group #2120
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[miga-heygen]

Reviewed as part of the 15-PR edit-panel redesign stack. Full stack review posted on #2120. No blockers on this PR. — Miga

[miguel-heygen]

Final current-head pass: ec3ad42. Reviewed the stack boundaries and current diff against the flat-inspector contracts; required checks have no failures/pending checks and no unresolved review threads remain. No new drift found; residual notes are non-blocking.

[miguel-heygen]

Superseding earlier approval: the latest max-review pass found confirmed correctness issues in this flat-inspector stack (identity/selector-index state, hide-all write races, timing inference, slider pointer/keyboard/reset semantics, and duplicate React keys). Hold merge and require fixes plus re-review on the current stack head.