ElmhurstSiteNotes had no representation for extensions: singular dimensions / walls / roof / floor fields could only describe the main bp. Summary PDFs lodge "1st Extension" / "2nd Extension" subsections in §4, §7, §8, §9 with optional "As Main: Yes" inheritance. This slice:
- Adds `ExtensionPart` dataclass and `ElmhurstSiteNotes.extensions: List[ExtensionPart]`.
- Adds `_split_section_by_bp` helper + per-bp parsing of dimensions / walls / roof / floor in the extractor; "As Main" inherits from the main bp.
- Refactors `_map_elmhurst_building_part` into a parameterised builder; adds `_map_elmhurst_building_parts` that yields Main + one SapBuildingPart per extension (capped at 4 per RdSAP10 §1.2).
- Scaffold test `test_summary_000474_mapper_produces_three_building_parts` flips from strict-xfail to passing.
Single-bp behaviour is unchanged (empty extensions list defaults). 752 existing tests stay green.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
