query summarizer: restrict deepest-node fallback to @target ancestors

[DanielCardonaRojas]

Summary

deepest_predicate_identifier (added in PR #222, crates/codemark-core/src/query/summarizer.rs) walks the entire query tree and returns the #eq?/#match? value bound to the deepest node anywhere:

fn walk(node, source, depth, best) {
    if let Some((_, val)) = predicate_capture_and_value(node, source)
        && best.as_ref().is_none_or(|(d, _)| depth >= *d) { *best = Some((depth, val)); }
    ...
}

A predicate in an unrelated sibling branch that happens to be nested deeper than the node enclosing @target can therefore outrank the true enclosing landmark, so the row shows an identifier from a branch that does not contain the target.

Impact

Low today: queries produced by the generator are a linear nested path (all predicates sit on the target or its ancestors' name fields), so there are no competing sibling predicates. It matters only for hand-crafted / hand-edited bookmark queries with branching predicate landmarks. short_display() is display-only, so the worst case is a slightly wrong list label — never a wrong match/resolution.

Fix

Restrict candidates to node patterns that are ancestors of the @target capture, then pick the deepest qualifying (enclosing) pattern. Add a test with a sibling predicate landmark to confirm it is ignored.

Raised by CodeRabbit on #222.