The global 'seen' set allowed anchored subtrees validated at a shallow
depth to be skipped when later referenced via alias at a greater depth.
This could let effective nesting exceed MaxYAMLDepth, enabling DoS.
Fix: replace the single 'seen' set with two tracking maps:
- validated (node -> min depth): only short-circuits when current depth
<= previously validated depth; re-checks at deeper contexts.
- visiting (node -> bool): per-path recursion stack for true cycle
detection (breaks alias loops without suppressing depth checks).
Add TestYAMLAliasDepthBypass that constructs a document with an
anchored 15-level subtree referenced via alias under 6 levels of
nesting, verifying the combined effective depth (22) is rejected.
Addresses security-review-bot findings on review #2774.
claw
9b1e93bfde