fix(studio): Style/Grade/Text parity gaps found in legacy-retirement audit

[vanceingalls]

What

Eighth PR in the flat inspector stack (see #2120 for the foundation and full stack list). Fixes 13 real functional gaps between the flat groups and the legacy components they replace — found by a thorough control-by-control audit (not just section-label matching) run after the legacy Style/Grade retirement (#2126) made these gaps directly user-visible for the first time.

Stack: #2120 → #2121 → #2122 → #2123 → #2124 → #2125 → #2126 → #2127 (this).

Why

The audit found these were pre-existing since each group was first built — the retained legacy sections (removed in #2126) had been silently covering for them the whole time. Two were hard drops (stroke color completely uneditable; per-corner border-radius unreachable from a uniform starting value) and one was a real behavioral bug (color grading sliders don't revive a grade parked at 0% strength, so edits look invisible). The rest were narrower gaps (unclamped/unenumerated stroke inputs, a missing uniform mask-inset slider, keyboard/blur safety on the grade compare button, HDR banner detail text, text-transform: capitalize unreachable, text-align: end unselectable, Size losing live-commit, Content textarea losing auto-focus, and non-numbered empty-field labels).

How

Three independent commits, each fixing one group's gaps against its legacy source of truth:

• Style (propertyPanelFlatStyleSections.tsx): stroke color now editable via a ColorField row; stroke width clamped 0–200px; stroke style validated against the 10 legacy options and exposed via a discoverable dropdown; per-corner radius now always reachable (deleted the dead branch that only mounted BorderRadiusEditor for already-non-uniform elements — it has its own unlink toggle and now always renders, matching legacy); restored the uniform mask-inset slider alongside the per-side fields.
• Grade (propertyPanelFlatColorGradingSection.tsx): added a visibleIntensity helper (identical to legacy) threaded into every Adjust/Vignette/Grain/Blur/Pixelate/LUT commit handler so editing a grade parked at 0% strength revives it to visible — the strength slider's own commit and preset application are correctly left untouched; the compare/hold button gained keyboard activation (Space/Enter) and window-blur release safety; the HDR banner and status indicator regained their informational detail text.
• Text (propertyPanelFlatTextSection.tsx, multi-field path): added the missing capitalize case option; aliased text-align: end onto the "right" button (matching how "start" already aliases onto "left"); restored live-commit on the Size field; wired key+autoFocus so switching or adding a text field focuses its Content textarea, matching legacy; empty fields now show a numbered "Text 1"/"Text 2" fallback instead of an indistinguishable bare "Text".

All three legacy source files (propertyPanelStyleSections.tsx, BorderRadiusEditor.tsx, propertyPanelColorGradingSection.tsx, propertyPanelColorGradingControls.tsx, propertyPanelSections.tsx) were read-only references — none were modified; they're still used by the non-flat legacy panel.

Test plan

• Every fix has a dedicated real DOM-event test with an exact assertion (drag/click/keyboard/blur simulation, not shallow snapshots), including the previously-unreachable per-corner-radius-from-uniform path and the revive-from-zero path, each independently verified in review by reproducing the exact prior bug and confirming the new test catches it.
• Full monorepo suite green (1582 tests, 0 failures across all three commits); oxlint/oxfmt/fallow clean on every commit.
• Unit tests added/updated
• Manual testing performed (this whole fix arc originated from live manual testing surfacing the Style/Grade duplication, which led to the audit that found these gaps)
• 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 ) 👈 (View in Graphite)
• 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
• feat(studio): flat inspector foundation + Text group #2120
• main

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