fix(splitter): re-initialize panels when child count changes dynamically

[YevheniiKotyrlo]

Defect Fixes

Fixes #8477

Splitter only calls initializePanels() in mounted(). When a SplitterPanel is added dynamically via v-if after mount (e.g., waiting for async data), the new panel never receives a flexBasis style — it renders at its natural flex size (~27%) instead of the configured :size percentage (40%).

PR #6394 added resetState() as a manual workaround, but consumers shouldn't need to call it — the Splitter should handle panel count changes automatically.

Related: #5463 (same root cause — closed with manual resetState workaround)

Problem

<Splitter>
  <SplitterPanel :size="60">Main content</SplitterPanel>
  <SplitterPanel v-if="detailLoaded" :size="40">
    <!-- Appears after async data loads -->
    <!-- Renders at ~27% instead of 40% — no flexBasis applied -->
  </SplitterPanel>
</Splitter>

After this fix, dynamically added panels immediately receive the correct flexBasis:

<Splitter>
  <SplitterPanel :size="60">Main content</SplitterPanel>
  <SplitterPanel v-if="detailLoaded" :size="40">
    <!-- Appears at exactly 40% — flexBasis applied on panel count change -->
  </SplitterPanel>
</Splitter>

Reproducer: bun run patch applies the fix, bun run unpatch reverts.

Root cause

initializePanels() queries this.$el.children for data-pc-name="splitterpanel" elements and sets flexBasis from panel.props.size. The panels computed (which iterates $slots.default()) correctly includes dynamically added panels, but initializePanels() never re-runs — the new DOM element has no flexBasis set.

Changes

packages/primevue/src/splitter/Splitter.vue: Add updated() lifecycle hook that tracks panel count via _lastPanelCount. When the count changes (panel added or removed), re-run initializePanels().

The count guard ensures this does not reset user resize state on every re-render — it only fires when panels are actually added or removed.

Scope / Impact

• No breaking changes — existing Splitters with static panels are unaffected (count never changes)
• Minimal change — reuses existing initializePanels(), adds only the updated() hook
• Fixes all dynamic panel patterns — v-if, dynamic components, programmatic panel addition/removal

Verification

Gate	Result
StackBlitz reproducer	bun run patch applies fix, bun run unpatch reverts
All 3 existing Splitter unit tests	PASS
Pressure tests (rapid toggle, remove, stateful, nested)	PASS
All CI gates (format, lint, test, build)	PASS