Function composeIfcx

Compose IFCX nodes into a flattened tree structure.

Algorithm:

1. Group all nodes by path (multiple nodes can reference same path)
2. Merge attributes (later wins - layer semantics)
3. Resolve inherits references (type-level data)
4. Build parent-child tree from children references
5. Apply tombstones: nodes whose strongest ifclite::deleted opinion is true are removed together with their subtrees (see tombstones.ts)