…level

The v2 (ontology) HTML parser -- the path the VLM partitioner uses via
partition_html(html_parser_version="v2") -> ontology_to_unstructured_elements --
set category_depth to the DOM/recursion nesting depth. That encoded physical
layout (Page -> Column -> block) rather than logical hierarchy, so every flat
sibling heading collapsed to the same depth and a multi-column page bumped every
element's depth. It diverged from the v1 parser and from the documented
metadata contract (depth derives from native heading levels h1/h2/h3).

Changes:
- Add shared helper `category_depth_from_html_tag` in partition/common/metadata.py:
  Title (ontology Title/Subtitle/Heading -> h1..h6) -> int(tag[1]) - 1
  (h1->0, h2->1, ...); ListItem -> enclosing ol/ul/dl count; else None.
- v1 parser `_category_depth` now delegates to the shared helper (DRY; behavior
  unchanged).
- v2 converter computes category_depth from the element's heading level (not
  nesting). Layout containers and non-heading content get None.

parent_id approach:
- Layout/container elements (Page, Column, Section, ...) keep their tree
  parent_id so the physical layout structure is preserved.
- Content elements are emitted with parent_id=None so the shared
  set_element_hierarchy post-processor (run by @apply_metadata on
  partition_html) assigns a heading-based parent -- subsections now chain under
  their enclosing section heading. (set_element_hierarchy skips elements that
  already have a parent_id, hence the None.)
- unstructured_elements_to_ontology (the reverse path) no longer reads content
  parent_id (now heading-based) to rebuild the tree; it reconstructs layout
  nesting from the layout-container elements' retained tree parent_id plus
  document order, via a container stack. Round-trip is preserved exactly.
- Inline-merge ("same level in the tree") previously reused category_depth as a
  nesting signal; it now uses a transient, non-serialized _ontology_nesting_depth
  attribute so heading-level category_depth doesn't change merge behavior.

Verified: category_depth follows heading level (h1->0, h2->1, h3->2); depth is
unaffected by multi-column layout; parent_id reflects section->subsection;
text_as_html, Page/Column containers, and table/form structure are preserved.
v2 now matches v1 on heading-level depth. Confirmed on the real Kaiser
EOC_CA_Individual-Family page 6 ("Cost Share Summary"->0, subsections->1,
"How to read..."->2, subsections chained under their section).

New tests cover a multi-level page and a multi-column page; existing v2, ontology
round-trip, and set_element_hierarchy tests stay green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>