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.

CLAUDE.md

@@ -69,18 +69,19 @@ The `mcp` FastMCP instance is configured with:
 - **Middleware stack**: `ErrorHandlingMiddleware` → `AuditMiddleware` (custom timing/audit log) → `TimingMiddleware` → `LoggingMiddleware`
 - **`_backend(ctx)`** helper: retrieves the backend from lifespan context, raises `ToolError` if not ready
 
-Tools are organized into 11 sections:
+Tools are organized into 12 sections:
 1. Drawing Management (11 tools): `drawing_*`
 2. Entity Creation (13 tools): `entity_create_*`
 3. Dimensions (5 tools): `dimension_*`
 4. Entity Modification (10 tools): `entity_move/copy/rotate/scale/mirror/offset/delete/array_*`
 5. Entity Query (3 tools): `entity_get`, `entity_list`, `entity_delete_many`
-6. Layer Management (12 tools): `layer_*`
+6. Layer Management (12 tools + 2 linetype_*): `layer_*`, `linetype_list`, `linetype_load`
 7. Block Operations (7 tools): `block_*`
 8. Analysis & Query (8 tools): `analysis_*`
 9. View & Screenshot (5 tools — includes `view_zoom_and_screenshot`): `view_*`
 10. Transactions (3 tools): `transaction_begin/commit/rollback`
 11. System (6 tools): `system_status/get_variable/set_variable/run_command/run_lisp/about`
+12. Engineering / Deterministic CAD (7 tools): `gear_draw_*`, `keyway_draw_*`, `titleblock_apply_iso_a3`, `drawing_finalize`
 
 ### Adding a New Tool
 
@@ -98,3 +99,5 @@ Tools are organized into 11 sections:
 - **COM backend only**: `system_run_command`, `system_run_lisp`, and `view_zoom_extents/window` (view ops are no-ops in ezdxf).
 - **Screenshot**: COM uses Win32 window capture (Pillow required); ezdxf renders via matplotlib.
 - **`_dc(obj)`**: converts dataclasses to dicts recursively for JSON serialization.
+- **Engineering layer scaffold**: `drawing_new` auto-bootstraps standard linetypes (CENTER, HIDDEN, PHANTOM) and engineering layers (GEOMETRY, DIM, CENTER, HIDDEN, PHANTOM, HATCH, TEXT, TITLEBLOCK). Pass `bootstrap=False` to opt out.
+- **Production drawings**: For real engineering output, use the `engineering/` package primitives via the `gear_*` / `keyway_*` / `titleblock_*` MCP tools — do NOT hand-draw teeth/keyways/sections with raw `entity_create_*` calls. Always end with `drawing_finalize` for the 8-step validator.