Implements Phase 7.1.2: StandardType -> BlockKind mapping for converting
walked StructElem nodes into the BlockKind taxonomy used by Phase 4 output.
Changes:
- Add BlockKind enum with all output block kinds (paragraph, heading with
level, table, list, list_item, figure, caption, code, block_quote, toc,
formula, reference, note, form_field_struct, inline, structural_container,
artifact, unknown)
- Add MappingResult struct bundling block_kind, is_emitted flag, and optional
diagnostic
- Add structure_type_to_block_kind() function for pure type mapping
- Add map_element_to_block() function as primary mapping API
- Add is_artifact() placeholder for Phase 3.4 marked-content integration
- Add 32 comprehensive unit tests covering all mapping paths
Key features:
- Complete type mapping for all 40+ PDF standard structure types
- Heading level extraction: H->level 1, H1..H6->level 1..6
- Inline elements (Span, Quote) map to Inline (not emitted as blocks)
- Structural containers (Document, Part, Sect, Div, etc.) map to
StructuralContainer (descend without emitting)
- Unknown types emit diagnostic and fall back to paragraph
Acceptance criteria:
- Every Standard structure type has a mapping decision
- Critical test: H1/H2 -> heading level 1/2
- Unit tests: list nesting, table grouping, span passthrough
- Unknown-type fallback path emits a diagnostic line
Refs: Plan section 7.1 lines 2552-2553
