fix(engine): use depth-based cursor restoration in Call/Return

[zharinov]

Summary

• Fix duplicate matches when sequences contain alternations (e.g., {[A B] @x}*)
• Change Call/Return from position-based to depth-based cursor restoration
• Preserve sibling advances from continue_search while restoring tree level when callee descended

Problem

When {[A B] @x}* iterates:

1. First iteration: Alt matches at lexical_declaration via continue_search
2. Return restores cursor to { (saved at Call time)
3. Repeat's Nav::Next from { goes to lexical_declaration again
4. Alt matches the same node → duplicate!

Solution

Instead of saving/restoring exact cursor position, save/restore tree depth:

• At Call: save cursor.depth()
• At Return: go up to saved depth (if descended) or stay put (if same level)

This preserves sibling advances while correctly restoring level after descending into children.

Test plan

• All 751 existing tests pass
• Bug reproduction now returns 2 matches instead of 3:

  cargo run -p plotnik-cli -- exec -q '
  Alt = [(lexical_declaration) (return_statement)]
  Q = (program (function_declaration body: (statement_block {(Alt) @a}* @all)))
  ' -s 'function f() { let x = 1; return x; }' -l javascript --entry Q