Preserve YAML sequence hierarchy depth in structured symbols

[Widthdom]

Summary

YAML paths such as jobs.build.steps[14].with.path are now stable, but every nested workflow member still reports depth:0.

Audit IDs: P-08. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Closed #3808 is adjacent structured-data outline work.

Expected behavior and PR scope

YAML mapping and sequence nodes must retain parent identity and correct depth through extraction and outline projection. Keep this PR YAML-specific unless a shared fix cleanly applies.

Implementation guide for Codex 5.6 sol xhigh

• Carry parent/depth metadata while descending mappings and indexed sequence elements.
• Define whether sequence element containers are explicit symbols or path-only nodes and test that contract.
• Cover root sequences, nested mappings/sequences, anchors/aliases, multiline scalars, and empty elements.
• Verify outline paths, parent IDs/names, ranges, sorting, and pagination.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• The representative deep path has the correct nonzero depth/parent chain.
• Sequence indexes and ranges remain stable.
• No duplicate/inflated symbols are introduced.

[Widthdom]

Work started on branch fix-issue4873.