Add deterministic engineering CAD layer (gear, keyway, titleblock, validator)

Introduces an engineering/ package that lets MCP clients produce production-
quality technical drawings without LLM-improvised geometry. Adds 7 new MCP
tools, an 8-step pre-completion validator, and an automatic linetype + layer
bootstrap on drawing_new so CENTER/HIDDEN/PHANTOM and standard mechanical
layers (GEOMETRY, DIM, HATCH, TEXT, TITLEBLOCK, ...) are always available.

- engineering/gear.py: pure-Python involute math, 40+ points/flank, helical
  + spur front views, deterministic section A-A (no helix lines in section).
- engineering/keyway.py: DIN 6885 keyway table + parametric keyed bore /
  side-section drawers.
- engineering/titleblock.py: ISO 7200 A3 (420x297 mm) title block; title
  text is written verbatim to the title cell.
- engineering/validator.py: DrawingValidator with 8 checks (real DIMENSION
  entities, no hidden orphans, linetypes loaded, bore + keyway in both
  views when expected, title consistency e.g. forbid 'HELICAL SPUR GEAR'
  when helix_angle truthy, saved-to-disk + file-on-disk, screenshot
  exported, dwg path returned).
- backends/ezdxf_backend.py: AutoCAD-only linetype fallback (HIDDEN,
  BORDER families) so ezdxf doc gets correct patterns when linetype_load
  is called for names ezdxf.tools.standards does not ship.
- backends/base.py: concrete helpers set_layer_active / ensure_linetypes.
- server.py: Section 12 with 7 engineering tools, drawing_new(bootstrap=
  True), and an ENGINEERING WORKFLOW block in the MCP instructions string.
- 30 new tests covering gear math, DIN6885 lookup, ISO A3 layout, title
  verbatim, and every validator check. Tool count 89 -> 96.

tests/test_engineering_gear.py

@@ -0,0 +1,128 @@
+"""Tests for engineering.gear — pure-math + drawing orchestration."""
+
+from __future__ import annotations
+
+import math
+
+import pytest
+
+from engineering.gear import (
+    draw_gear_section_aa,
+    draw_helical_gear_front_view,
+    generate_full_gear_outline,
+    generate_involute_flank,
+    involute_xy,
+)
+from engineering.layers import (
+    ensure_engineering_layers,
+    ensure_standard_linetypes,
+)
+
+
+# ---------------------------------------------------------------------------
+# Pure math (sync)
+# ---------------------------------------------------------------------------
+
+
+def test_involute_xy_at_zero_returns_base_point():
+    x, y = involute_xy(10.0, 0.0)
+    assert math.isclose(x, 10.0, abs_tol=1e-9)
+    assert math.isclose(y, 0.0, abs_tol=1e-9)
+
+
+def test_generate_involute_flank_min_30_points():
+    flank = generate_involute_flank(base_r=20.0, root_r=18.0, outer_r=25.0, n_points=5)
+    assert len(flank) >= 30
+
+
+def test_generate_involute_flank_radius_progression():
+    base_r, root_r, outer_r = 20.0, 18.0, 25.0
+    flank = generate_involute_flank(base_r=base_r, root_r=root_r, outer_r=outer_r)
+    first_r = math.hypot(*flank[0])
+    last_r = math.hypot(*flank[-1])
+    assert math.isclose(first_r, root_r, abs_tol=1e-6)
+    assert math.isclose(last_r, outer_r, rel_tol=1e-3)
+
+
+def test_generate_full_gear_outline_closed():
+    pts = generate_full_gear_outline(module=3.0, teeth=24)
+    assert pts[0] == pts[-1]
+
+
+def test_generate_full_gear_outline_point_count():
+    teeth = 24
+    pts = generate_full_gear_outline(module=3.0, teeth=teeth)
+    # At minimum: teeth * (40 right + 40 left) = 1920 unique flank points,
+    # plus tip and root arc samples. Closing duplicate adds 1.
+    assert len(pts) >= teeth * 40
+
+
+# ---------------------------------------------------------------------------
+# Drawing orchestration (async, ezdxf backend)
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_helical_gear_front_view_returns_metadata(backend):
+    await ensure_standard_linetypes(backend)
+    await ensure_engineering_layers(backend)
+    res = await draw_helical_gear_front_view(
+        backend, module=3, teeth=24, helix_angle=20.0, hand="RH",
+        center=(0.0, 0.0),
+    )
+    expected_keys = {
+        "outline", "pitch_circle", "base_circle", "outer_circle", "root_circle",
+        "helix_symbol", "helix_label", "bore", "keyway", "metadata",
+    }
+    assert expected_keys.issubset(res.keys())
+    md = res["metadata"]
+    for k in (
+        "module", "teeth", "pressure_angle", "helix_angle", "hand", "center",
+        "bore_diameter", "keyway_width", "keyway_depth",
+        "pitch_radius", "outer_radius", "base_radius", "root_radius",
+    ):
+        assert k in md, f"metadata missing key {k!r}"
+
+
+@pytest.mark.asyncio
+async def test_helical_gear_with_bore_and_keyway_handles_present(backend):
+    await ensure_standard_linetypes(backend)
+    await ensure_engineering_layers(backend)
+    res = await draw_helical_gear_front_view(
+        backend, module=3, teeth=24, helix_angle=20.0,
+        bore_diameter=25.0, keyway_width=8.0, keyway_depth=4.0,
+    )
+    assert res["bore"] is not None
+    assert res["keyway"] is not None
+    assert "bore" in res["keyway"]
+    assert "keyway_polyline" in res["keyway"]
+
+
+@pytest.mark.asyncio
+async def test_helical_gear_metadata_radii_correct(backend):
+    await ensure_standard_linetypes(backend)
+    await ensure_engineering_layers(backend)
+    res = await draw_helical_gear_front_view(
+        backend, module=3, teeth=24, helix_angle=20.0, pressure_angle=20.0,
+    )
+    md = res["metadata"]
+    assert math.isclose(md["pitch_radius"], 36.0, abs_tol=1e-6)
+    assert math.isclose(md["outer_radius"], 39.0, abs_tol=1e-6)
+    assert math.isclose(md["base_radius"], 36.0 * math.cos(math.radians(20.0)), abs_tol=1e-3)
+    assert math.isclose(md["root_radius"], 32.25, abs_tol=1e-6)
+
+
+@pytest.mark.asyncio
+async def test_section_aa_does_not_draw_helix_lines(backend):
+    await ensure_standard_linetypes(backend)
+    await ensure_engineering_layers(backend)
+    front = await draw_helical_gear_front_view(
+        backend, module=3, teeth=24, helix_angle=20.0,
+        bore_diameter=25.0, keyway_width=8.0, keyway_depth=4.0,
+    )
+    section = await draw_gear_section_aa(
+        backend, gear_metadata=front["metadata"],
+        x_offset=200.0, face_width=40.0,
+    )
+    assert "helix" not in section
+    assert "helix_symbol" not in section