writes: name the snapshot-clearing rule as invalidateSourceRows()

The two-line staleness pair was copied in setElement, __unset, and the
deprecated offsetUnset, with the rationale comment repeated at each site.
One private helper now holds both lines and the why; a future write path
calls it instead of having to notice and copy the pattern. Missing it
means derived sets serve pre-write data from their carried snapshots.

Author: daveedis

src/Deprecations.php

@@ -535,8 +535,7 @@ public function offsetUnset(mixed $offset): void
     {
         $offset = self::coerceOffset($offset);
         self::triggerArrayAccessDeprecation($offset, 'unset');
-        $this->sourceRows       = null;   // same staleness rule as setElement()
-        $this->root->sourceRows = null;
+        $this->invalidateSourceRows();
         unset($this->data[$offset]);
     }
 

src/SmartArrayBase.php

@@ -374,18 +374,25 @@ public function at(int|SmartString|SmartNull $index): static|SmartNull|SmartStri
         };
     }
 
+    /**
+     * A write makes kept source rows stale: clears this array's snapshot and its result
+     * set's ($this->root is the result set on rows, $this on top-level arrays). Every
+     * write path must call this, or toArray() can serve pre-write data (see $sourceRows).
+     */
+    private function invalidateSourceRows(): void
+    {
+        $this->sourceRows       = null;
+        $this->root->sourceRows = null;
+    }
+
     /**
      * Stores an element with automatic type conversion.
      * Scalars and nulls are stored as-is; arrays are converted to SmartArray instances.
      * Smart values are unwrapped first, so `$a->key = $b->key` works in any mode.
      */
     private function setElement(int|string|null $key, mixed $value): void
     {
-        // A write makes kept source rows stale: this array's own copy, and its result
-        // set's when this is a row ($this->root is the result set on rows, $this on
-        // top-level arrays)
-        $this->sourceRows       = null;
-        $this->root->sourceRows = null;
+        $this->invalidateSourceRows();
 
         // Unwrap Smart values (SmartString, SmartArray, SmartNull) to their raw
         // equivalents; nested arrays then convert to this array's mode below.
@@ -1436,8 +1443,7 @@ public function __isset(string $name): bool
      */
     public function __unset(string $name): void
     {
-        $this->sourceRows       = null;   // same staleness rule as setElement()
-        $this->root->sourceRows = null;
+        $this->invalidateSourceRows();
         unset($this->data[$name]);
     }