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_titleblock.py

@@ -0,0 +1,104 @@
+"""Tests for engineering.titleblock — ISO 7200 A3 layout."""
+
+import pytest
+
+from engineering.titleblock import TitleBlockMetadata, apply_iso_a3_titleblock
+
+pytestmark = pytest.mark.asyncio
+
+
+def _meta(**overrides) -> TitleBlockMetadata:
+    base = dict(
+        title="HELICAL GEAR",
+        drawing_no="AM-2026-001",
+        part_no="GR-24T-M3",
+        material="C45",
+        scale="1:1",
+        units="mm",
+        drawn_by="Umutcan",
+        checked_by="QA",
+        date="2026-04-28",
+        sheet="1/1",
+        revision="A",
+        company="Anka-Makine",
+    )
+    base.update(overrides)
+    return TitleBlockMetadata(**base)
+
+
+async def test_titleblock_outer_border_dimensions(backend):
+    """Outer border bbox must equal A3 sheet (420 x 297 mm)."""
+    result = await apply_iso_a3_titleblock(backend, metadata=_meta())
+    bbox = result["bbox"]
+    width = bbox["max"][0] - bbox["min"][0]
+    height = bbox["max"][1] - bbox["min"][1]
+    assert width == pytest.approx(420.0)
+    assert height == pytest.approx(297.0)
+
+
+async def test_title_text_exact_match(backend):
+    """Title text in the drawing must equal metadata.title verbatim."""
+    result = await apply_iso_a3_titleblock(
+        backend, metadata=_meta(title="HELICAL GEAR"),
+    )
+    title_ent = await backend.entity_get(result["title_text"])
+    assert title_ent.type == "TEXT"
+    assert title_ent.properties["text"] == "HELICAL GEAR"
+
+
+async def test_titleblock_creates_text_for_each_metadata_field(backend):
+    """value_texts must contain a handle for every metadata field."""
+    result = await apply_iso_a3_titleblock(backend, metadata=_meta())
+    expected_keys = {
+        "drawing_no", "revision", "sheet",
+        "part_no", "material", "scale",
+        "drawn_by", "checked_by", "date", "company",
+    }
+    assert set(result["value_texts"].keys()) == expected_keys
+    # All handles must resolve to TEXT entities
+    for key, handle in result["value_texts"].items():
+        ent = await backend.entity_get(handle)
+        assert ent.type == "TEXT", f"{key} expected TEXT, got {ent.type}"
+
+
+async def test_titleblock_origin_offset(backend):
+    """Passing origin=(100,50) must shift the bbox by that amount."""
+    result = await apply_iso_a3_titleblock(
+        backend, metadata=_meta(), origin=(100.0, 50.0),
+    )
+    bbox = result["bbox"]
+    assert bbox["min"][0] == pytest.approx(100.0)
+    assert bbox["min"][1] == pytest.approx(50.0)
+    assert bbox["max"][0] == pytest.approx(520.0)
+    assert bbox["max"][1] == pytest.approx(347.0)
+
+
+async def test_titleblock_borders_on_titleblock_layer(backend):
+    """Outer + inner borders + grid lines must be on TITLEBLOCK layer."""
+    result = await apply_iso_a3_titleblock(backend, metadata=_meta())
+    outer = await backend.entity_get(result["outer_border"])
+    inner = await backend.entity_get(result["inner_border"])
+    assert outer.layer == "TITLEBLOCK"
+    assert inner.layer == "TITLEBLOCK"
+    for handle in result["titleblock_lines"]:
+        ent = await backend.entity_get(handle)
+        assert ent.layer == "TITLEBLOCK"
+
+
+async def test_titleblock_text_on_text_layer(backend):
+    """Title and all value texts must be on TEXT layer."""
+    result = await apply_iso_a3_titleblock(backend, metadata=_meta())
+    title = await backend.entity_get(result["title_text"])
+    assert title.layer == "TEXT"
+    for handle in result["value_texts"].values():
+        ent = await backend.entity_get(handle)
+        assert ent.layer == "TEXT"
+
+
+async def test_titleblock_metadata_in_result(backend):
+    """Returned metadata dict must round-trip the input metadata."""
+    md = _meta(title="SPUR GEAR", drawing_no="AM-2026-002")
+    result = await apply_iso_a3_titleblock(backend, metadata=md)
+    assert result["metadata"]["title"] == "SPUR GEAR"
+    assert result["metadata"]["drawing_no"] == "AM-2026-002"
+    assert result["metadata"]["company"] == "Anka-Makine"