feat(arch): door and window symbols, straight/L/U stairs and the Blondel check

openings.py draws what stands in the gap the wall engine cuts, from the same
numbers - the wall axis, its face offsets and model.segment_of - so the symbol
cannot drift off its gap. A door is drawn open at 90 degrees: its leaf as long
as the opening is wide, square to the wall from the hinge on the swing-side
face, and a quarter arc from the closed position to the open one. swing=in is
the left of the axis (the inside of a CCW perimeter); hand names the hinge jamb
seen from the swing side facing the wall. All four swing/hand pairs are tested
against hand-computed hinges and arc angles. A window is a frame line on each
face and two glazing lines at the thirds of the thickness.

stairs.py draws every riser once as a line across its flight, the outline, a
walking line up the middle with an open arrow at the top, and the language's
'up' word on the first tread at 2.5 mm times the plot scale. An L or a U turns
on a square landing; its first flight carries (risers + 1) // 2 risers. The
riser count and every dimension are refused by name before any geometry.

blondel() reports 2R + G against 600-650 mm - the step-length rule attributed
to Francois Blondel (1675), reported as a design rule and never as a building
code: no national stair regulation is checked.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

engineering/arch/__init__.py

@@ -30,6 +30,16 @@ from .model import (
     wall_from_dict,
     wall_length,
 )
+from .openings import opening_prims
+from .stairs import (
+    ARROW_PAPER_MM,
+    BLONDEL_RANGE,
+    STAIR_KINDS,
+    TEXT_PAPER_MM,
+    TURNS,
+    blondel,
+    stair_prims,
+)
 from .walls import (
     MIN_JOIN_ANGLE_DEG,
     WallGeometry,
@@ -46,17 +56,23 @@ __all__ = [
     "ARCH_LAYER_DEFS",
     "ARCH_LAYER_ROLES",
     "ARCH_ROLE_LAYER",
+    "ARROW_PAPER_MM",
+    "BLONDEL_RANGE",
     "KINDS",
     "LANGS",
     "MIN_JOIN_ANGLE_DEG",
     "Opening",
     "Room",
+    "STAIR_KINDS",
     "Stair",
+    "TEXT_PAPER_MM",
+    "TURNS",
     "WALL_HATCH_TABLE",
     "WALL_MATERIALS",
     "Wall",
     "WallGeometry",
     "axis_segments",
+    "blondel",
     "decode",
     "encode",
     "face_offsets",
@@ -64,10 +80,12 @@ __all__ = [
     "fmt_number",
     "from_payload",
     "opening_from_dict",
+    "opening_prims",
     "room_from_dict",
     "room_segments",
     "segment_of",
     "stair_from_dict",
+    "stair_prims",
     "tag_prefix",
     "to_payload",
     "to_values",