fix: guard overlay timestamp assignment against undefined in pressed move

[NemeZZiZZ]

Problem

OverlayImp.eventPressedPointMove assigns point.timestamp unconditionally,
while the sibling fields (dataIndex, value) are guarded with isNumber:

// src/component/Overlay.ts
eventPressedPointMove(point: Partial<Point>, pointIndex: number): void {
  this.points[pointIndex].timestamp = point.timestamp   // ❌ no guard
  if (isNumber(point.dataIndex)) {                      // ✓ guarded
    this.points[pointIndex].dataIndex = point.dataIndex
  }
  if (isNumber(point.value)) {                          // ✓ guarded
    this.points[pointIndex].value = point.value
  }
  ...
}

point is built by OverlayView._coordinateToPoint, which sets
timestamp = chartStore.dataIndexToTimestamp(dataIndex) ?? undefined.
dataIndexToTimestamp returns null when dataList is empty or the lookup
fails and no period is configured (src/Store.ts:950). In that case
point.timestamp is undefined, and this line overwrites a previously valid
timestamp on the dragged point — dropping the point from its time-based
placement for overlays placed by { timestamp, value }.

Fix

Guard the assignment the same way the sibling fields (and the other move
handlers in this file) are guarded:

   eventPressedPointMove(point: Partial<Point>, pointIndex: number): void {
-    this.points[pointIndex].timestamp = point.timestamp
+    if (isNumber(point.timestamp)) {
+      this.points[pointIndex].timestamp = point.timestamp
+    }
     if (isNumber(point.dataIndex)) {

The same if (isNumber(point.timestamp)) guard is already used by the two
neighbouring move handlers in this file (continuousDrawingModeEventMoveForDrawing
and stepDrawingModeEventMoveForDrawing), so this brings eventPressedPointMove
in line with them.

Verification

• pnpm type-check — pass (exit 0)
• pnpm build-esm (runs code-lint via biome, then ESM build) — pass (154 files checked, no fixes, exit 0)

Notes

• Only the timestamp assignment is guarded; dataIndex/value handling and the
  performEventPressedMove callback are untouched.
• No public API changes.