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.

backends/base.py

@@ -358,6 +358,15 @@ class AutoCADBackend(ABC):
     @abstractmethod
     async def layer_show(self, name: str) -> dict: ...
 
+    # ── linetype management ──────────────────────────────────────────────────
+    @abstractmethod
+    async def linetype_list(self) -> list[str]: ...
+
+    @abstractmethod
+    async def linetype_load(
+        self, name: str, file: str | None = None,
+    ) -> dict: ...
+
     # ── block operations ─────────────────────────────────────────────────────
     @abstractmethod
     async def block_list(self) -> list[BlockInfo]: ...
@@ -449,6 +458,33 @@ class AutoCADBackend(ABC):
     @abstractmethod
     async def system_run_lisp(self, expression: str) -> dict: ...
 
+    # ── concrete helpers (use existing primitives) ──────────────────────────
+    async def set_layer_active(self, name: str) -> None:
+        """Convenience: alias for layer_set_current. Concrete; backends need not override."""
+        await self.layer_set_current(name)
+
+    async def ensure_linetypes(
+        self, names: list[str], file: str | None = None,
+    ) -> dict[str, str]:
+        """Idempotently load each linetype in `names`. Returns {name: status}.
+        Status is one of: 'already_loaded', 'loaded', 'failed: <msg>'.
+        """
+        try:
+            current = {ln.lower() for ln in await self.linetype_list()}
+        except Exception:
+            current = set()
+        results: dict[str, str] = {}
+        for n in names:
+            if n.lower() in current:
+                results[n] = "already_loaded"
+                continue
+            try:
+                await self.linetype_load(n, file=file)
+                results[n] = "loaded"
+            except Exception as exc:
+                results[n] = f"failed: {exc}"
+        return results
+
 
 # ---------------------------------------------------------------------------
 # Helpers