STEP export: AP242 file of our own, names, colours, components as assemblies

Our writer assembles the file: AP242 header and contexts, a product per part
(or per component, with NEXT_ASSEMBLY_USAGE_OCCURRENCE and item-defined
placements per occurrence), named solids, surface colours as styled items,
ADVANCED_FACE for advanced breps. truck now writes only each body's B-rep,
which we copy in; identical geometric entities are written once and shared.
Designs with components export as assemblies (bodies in component frames).

step_validate also rejects unreferenced non-root entities. Tests: the sample
plate and a component assembly export, validate and read back with no
warnings and the same volume, area and face counts; the oracle replay checks
area and faces too (39 rebuildable cases pass). An independent Part 21
parser (step-p21, run outside the repo) accepts our exports.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

README.md

@@ -29,7 +29,8 @@ storytold "craft" family.
   changed feature; it can be rolled back, and features can be suppressed, renamed, edited and
   deleted.
 - **Parameters** with unit-aware expressions (`2 * width + 5 mm`, `angle / 2`).
-- **Files**: designs as JSON (`.solvecraft`), export to STEP, 3MF, STL (binary/ASCII) and OBJ;
+- **Files**: designs as JSON (`.solvecraft`), export to STEP (AP242 with body names, colours and
+  components as assemblies), 3MF, STL (binary/ASCII) and OBJ;
   3MF and STL import as mesh bodies (they render, measure, move and export; solid features need
   B-rep bodies);
   STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) as a base feature