Add validation, traversal utilities and additional placement strategies

[mass2527]

• Add layout validation utilities (validateLayout, isValidLayoutNode,
  isValidPanelNode, isValidSplitNode) for validating layouts loaded from
  external sources like localStorage
• Add layout traversal utilities (traverseLayout, findAllPanels,
  findAllSplits, countPanels, findNodeById, findPanelById, getPanelIds,
  getTreeDepth) for common layout operations
• Add additional placement strategies:
  • lastPanelRightStrategy: adds panels to the right of the rightmost panel
  • lastPanelBottomStrategy: adds panels below the bottommost panel
  • equalHeightBottomStrategy: maintains equal heights when adding panels
  • createFixedPlacementStrategy: factory for custom fixed placement
• Add comprehensive tests for all new utilities (96 new tests)

https://claude.ai/code/session_016RbWKu8BS3dCvPnovr6Ysc

[claude]

Code review

Found 1 issue that needs attention:

---

src/validation.ts (line 355-360)

Issue: The isValidSplitNode function has a type safety bug - it accepts split nodes with null children, even though SplitNode.left and SplitNode.right are typed as LayoutNode (not nullable).

Root cause: isValidLayoutNode returns true for null values (see line 277-279 where it returns node is LayoutNode | null). This means a split node like this would incorrectly pass validation:

{
  type: "split",
  id: "split-1",
  orientation: "horizontal",
  ratio: 0.5,
  left: null,  // Invalid! Should be LayoutNode, not null
  right: { type: "panel", id: "panel-1" }
}

Suggested fix: Add explicit null checks before validating with isValidLayoutNode:

if (!("left" in nodeObj) || nodeObj.left === null || !isValidLayoutNode(nodeObj.left)) {
  return false;
}

if (!("right" in nodeObj) || nodeObj.right === null || !isValidLayoutNode(nodeObj.right)) {
  return false;
}

This ensures the type guard correctly narrows to SplitNode only when both children are non-null LayoutNode values.

Reference: src/validation.ts#L354-L361