feat(mech): shared foundation - primitives, XDATA codec, standards registry, ISO 128-50 materials

engineering/mech/ opens with the four modules every track-B/G branch imports,
so the four worktrees can be cut from this commit and none of them has to
invent a name the others will conflict with.

- primitives.py: the typed descriptions the pure modules emit (Line, Arc,
  Circle, Poly, HatchArea, Text, DimIntent) with bbox / translate / rotate /
  scale, and ROLE_LAYER - the one place an ISO 128 line role becomes a layer
  name, so no module hardcodes a layer. The arc bbox sweeps its quadrants: a
  0-180 arc reports y=+r, not its endpoints, which is what keeps the view
  layout from stacking two views on top of each other.
- xdata.py: the ACADMCP_MECH codec in Track A's shape - a 1001 application
  marker, 1000 chunks of at most 255 ASCII characters, the 16 KB per-entity
  ceiling enforced before anything is written, and a foreign application, a
  corrupt chunk stream, a wrong version or an unknown kind refused with
  ValueError instead of decoded into something plausible.
- standards/__init__.py: the table registry. A size outside the coverage is
  refused against the coverage; a size inside it that is not a listed row is
  refused against the rows. Nothing is interpolated, an ambiguous variant is
  never resolved by taking the first row, and a registration whose SOURCE does
  not name the table it was transcribed from is refused.
- standards/materials.py: the section-hatch map for nine materials. ISO
  128-50 standardises only the general hatching (narrow continuous lines at 45
  degrees) - that is acadiso.pat ANSI31 and what engineering/gear.py already
  draws for steel. The distinguishing symbols are ASME Y14.2M's section lining
  as shipped in AutoCAD 2026's acadiso.pat, read off the installed file; cast
  iron's departure from ASME and the absence of any mechanical symbol for
  concrete and wood are stated in the rows themselves. Angles are 0.0 because
  the patterns already carry 45 degrees, and the two architectural patterns'
  scales are derived from the ANSI31 line spacing and recomputed by the test
  rather than chosen.

No tool is registered here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

engineering/mech/__init__.py

@@ -0,0 +1,49 @@
+"""The mechanical part model, view engine and standards tables.
+
+Merge rule for the four track-B/G branches: the re-exports below are additive
+and alphabetical — keep every side's names.
+"""
+
+from __future__ import annotations
+
+from .primitives import (
+    ROLE_LAYER,
+    ROLES,
+    Arc,
+    Circle,
+    DimIntent,
+    HatchArea,
+    Line,
+    Poly,
+    Prim,
+    Pt,
+    Role,
+    Text,
+    bbox,
+    layer_for,
+    points_of,
+    rotate,
+    scale,
+    translate,
+)
+
+__all__ = [
+    "ROLES",
+    "ROLE_LAYER",
+    "Arc",
+    "Circle",
+    "DimIntent",
+    "HatchArea",
+    "Line",
+    "Poly",
+    "Prim",
+    "Pt",
+    "Role",
+    "Text",
+    "bbox",
+    "layer_for",
+    "points_of",
+    "rotate",
+    "scale",
+    "translate",
+]