Optimize form editor subscribe callback to prevent unnecessary operations

[Copilot]

The subscribe callback was executing enforceBlockNesting, enforceBlockSelection, and lockFormBlock on every editor state change, causing performance issues during editing.

Changes

• State tracking: Added tracking for root block IDs, selected block ID, and lock status to detect actual changes
• Conditional execution: Operations now only run when their relevant state changes:
  • enforceBlockNesting - only when root blocks are added, removed, or reordered
  • enforceBlockSelection - only when selection changes
  • lockFormBlock - only once, verified by checking lock attributes directly

Before/After

Before:

if (isCurrentPostTypeJetpackForm) {
  locateFormBlock();
  enforceBlockNesting();      // Every state change
  enforceBlockSelection();    // Every state change
  lockFormBlock();            // Every state change
}

After:

if (isCurrentPostTypeJetpackForm) {
  if (!formBlockClientId) locateFormBlock();
  
  const currentRootBlockIds = JSON.stringify(rootBlocks.map(b => b.clientId));
  if (currentRootBlockIds !== lastRootBlockIds) {
    enforceBlockNesting();    // Only when root blocks change
  }
  
  if (currentSelectedBlockId !== lastSelectedBlockId) {
    enforceBlockSelection();  // Only when selection changes
  }
  
  if (!isFormBlockLocked && formBlockClientId) {
    lockFormBlock();          // Only once
    // Verify via attributes
  }
}

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