fix(studio): widen FlatSlider's click/drag hit area vertically

[vanceingalls]

What

Twelfth PR in the flat inspector stack (see #2120 for the foundation and full stack list). Widens FlatSlider's click/drag hit area from 2px (matching the thin visible line) to 20px, without changing anything about how the slider looks.

Stack: #2120 → ... → #2136 → #2142 (this).

Why

Requested directly after using the redesigned inspector live — the slider's clickable/draggable region was exactly as tall as its thin visual line, making it easy to miss.

How

• The visible 2px line is now a decorative child, vertically centered inside a taller (20px) wrapping element.
• The wrapping element carries the role, aria-* attributes, onPointerDown handler, and cursor styling that used to live on the thin line itself.
• The click-ratio math only reads left/width from the target element's bounding rect (never height), so click-position accuracy is completely unaffected by the height change.
• Knob and center-tick positioning (top-1/2 -translate-y-1/2) already centered relative to the parent's height, so they needed no changes to stay correctly aligned.

Test plan

• Existing click-accuracy test passes unmodified (same rect-mocking technique, still asserts exact committed values).
• New test asserts a pointerdown near the edge of the widened hit box (not the center) still commits the correct value — this is the specific behavior being fixed.
• Verified live in the browser: measured the actual rendered hit area (20px) vs. visible line (2px), then dispatched real pointer events at both the top and bottom edges of the widened area and confirmed the committed value changed correctly each time (previously, only the 2px center would have registered).
• Full monorepo suite green (1582 tests, 0 failures); 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): resolve flat-inspector review defects #2225
• fix(studio): debounce flat slider drag commits to prevent rapid-fire writes #2190
• fix(studio): support drag on FlatSlider, not just click-to-set #2186
• fix(studio): widen FlatSlider's click/drag hit area vertically #2142 👈 (View in Graphite)
• feat(studio): animate flat inspector group expand/collapse #2136
• refactor(studio): remove section pinning from the flat inspector #2135
• refactor(studio): scroll only the open group's body, keep headers fixed #2134
• fix(studio): Style/Grade/Text parity gaps found in legacy-retirement audit #2127 : 2 other dependent PRs (#2128 , #2129 )
• 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

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: a88b530. 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.