feat(scraps): Adopt SplitPanel with composable API redesign

[priscilawebdev]

Adopts SplitPanel into @sentry/scraps/splitPanel and redesigns its API in the same change. The old prop-config shape was clunky enough at the call site (every caller had to thread availableSize in by hand and pass a left/right/top/bottom config object) that promoting it as-is would have locked the suboptimal shape into the design system. Better to redesign now, while there are only two consumers.

Two precedents in the same vein: #113489 (feat(scraps): adopt GlobalDrawer component into design system) collapsed boolean props into mode and dropped unused overrides during its promotion, for the same reason.

The new API

<SplitPanel orientation="horizontal" sizeStorageKey="conv-split">
  <SplitPanel.Panel defaultSize={300} minSize={200} maxSize={600}>
    <LeftContent />
  </SplitPanel.Panel>
  <SplitPanel.Divider />
  <SplitPanel.Panel>
    <RightContent />
  </SplitPanel.Panel>
</SplitPanel>

• Composable — <SplitPanel.Panel> and <SplitPanel.Divider> as subcomponents instead of left={{...}} right={...} prop bags.
• Self-measuring — the root attaches the ref to its own outer element and uses useDimensions internally. Callers drop the useRef + useDimensions + width > 0 gate pattern.
• A11y baked in — <SplitPanel.Divider> renders with role=\"separator\", aria-orientation, aria-valuemin/max/now, and arrow-key resize (Shift for a coarser step, Home / End to snap to bounds).
• Custom dividers via useSplitPanelDivider — for the 1px-line look the conversation layout wants, the consumer renders its own styled element and spreads props from the hook (ARIA + event handlers + tabIndex + role).
• Imperative controls via useSplitPanel — descendants can call maximiseSize / minimiseSize / resetSize and read isMaximized / isMinimized.
• Breaking change — no shim for the old API. Both real consumers are migrated in this PR.

Scope decisions

• 2-panel only. Both consumers are 2-panel; N-panel adds size-redistribution math, ResizeTrigger IDs, and per-panel size tracking with no current consumer to validate against. The composable shape keeps the door open for N-panel later without rewriting the surface.
• Keyboard resize on the divider. Arrow keys nudge by 10px (Shift → 50px); Home / End snap to minSize / maxSize. Standard separator a11y pattern.
• availableSize survives only as an internal detail of the ReplaySplitPanel wrapper, where it's used to compute the resize end position as a percentage for analytics. The Replay layout already measures itself for its own grid, so threading the measurement avoids a redundant second useDimensions.

Verification

• pnpm lint:js (touched files): clean
• pnpm typecheck: clean
• pnpm test-ci splitPanel.spec.tsx: 6 / 6 (covers horizontal and vertical orientations, collapse-to-fill behavior with DOM-identity preservation across rerenders, and the divider's ARIA attributes)

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.58%	93.58%	±0%
Typed	133,106	133,150	🟢 +44
Untyped	9,131	9,131	±0

🔍 4 new type safety issues introduced

any-typed symbols (2 new)

File	Line	Detail
static/app/components/core/splitPanel/splitPanel.tsx	173	child (param)
static/app/components/core/splitPanel/splitPanel.tsx	316	child (param)

Type assertions (as) (2 new)

File	Line	Detail
static/app/components/core/splitPanel/splitPanel.tsx	178	as SplitPanelPanelProps — child.props as SplitPanelPanelProps
static/app/components/core/splitPanel/splitPanel.tsx	318	as SplitPanelPanelProps — child.props as SplitPanelPanelProps

This is informational only and does not block the PR.