feat(mech): SECTION 23 completes with centre_marks, section_line, hatch_material

centre_marks draws ISO 128-23 centre marks or full centre lines and reads
each circle's real centre and radius back out of the drawing through
entity_get instead of being told them - a radius restated from memory is
the silent wrong number this repository keeps refusing to ship. A handle
that is not a CIRCLE or ARC, and a circle in a plane tilted out of WCS XY
(which has no radius in this frame), are both refused by handle before
anything is written. The dash proportions are not transcribed here: the
prims are plain lines on CENTER, whose linetype already carries them.

section_line draws the ISO 128-40 cutting-plane line - wide segments at
the ends on GEOMETRY, the thin long-dash-dot middle on CENTER, an arrow at
each end whose head sits on the line end and points the way the section is
viewed, the same letter beyond both arrows - and returns the five-key
plane dict {p1, p2, label, direction, style} that mech_view_add(kind=
"section") consumes, so the label on the plan and the label on the view
are the same string by construction. A viewing direction parallel to the
plane is refused rather than drawn.

hatch_material hatches through standards.materials.hatch_for, so the
ISO 128-50 map and its refusal stay in one place; scale multiplies the
material's own pattern scale and angle replaces its angle. An unknown
material is refused with the list before any entity is written.

DEVIATION - one defect in the plan's own draw_material_hatch, fixed here
with two tests that fail against the plan's code. It called
entity_create_hatch with ROLE_LAYER["hatch"] without creating that layer,
the same omission review found in Task 10's draw_annotation_prims.
MEASURED live on AutoCAD 2026 (probe: GetActiveObject, Documents.Add(),
doc.Close(False) in a finally): on a scratch document whose layer table is
['0'], msp.AddHatch(0, "ANSI31", False) + AppendOuterLoop + Evaluate()
leaves the AcDbHatch in model space, and the very next statement
hatch.Layer = "HATCH" - which is the one line _apply_entity_attrs runs -
raises com_error (-2147352567, ..., 'Key not found', ..., -2145386476);
model space then still reports [('AcDbHatch', '0')] and the layer table is
still ['0'], so the refusal leaves an orphan hatch on layer 0. After
doc.Layers.Add("HATCH") the same assignment succeeds. drawing_open runs no
bootstrap, which is exactly when a cut face has no HATCH layer to land on.
draw_material_hatch now calls ensure_annotation_layers before creating the
hatch and reports layer / layers_created, as draw_annotation_prims does.
The whole task re-measured live afterwards through ComBackend on a fresh
scratch document (layer table ['0']): hatch_material -> {'ok': True,
'handle': '82', 'pattern': 'ANSI31', 'layer': 'HATCH', 'layers_created':
['HATCH']} with the entity read back as HATCH on HATCH; centre_marks on a
live CIRCLE r=12.5 -> sources [{'handle': '84', 'center': [80.0, 0.0],
'radius': 12.5}]; section_line -> 9 entities and the five-key plane; layer
table after ['0', 'CENTER', 'GEOMETRY', 'HATCH', 'TEXT']; document closed.

tests/test_mech_marks.py: 17 passed. Gates: ruff check clean, inventory
regenerated to 209 tools, test_tool_registry (engineering 12 -> 15, total
206 -> 209), test_discovery_aliases, test_tool_inventory,
test_capability_contract and test_release_consistency all pass. Full
suite: 3343 passed, the 6 pre-existing failures verified identical with
this change stashed, and
test_release_consistency.py::test_document_numbers_match_the_tree
deselected as Global Constraints instructs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

docs/tool-inventory.json

@@ -3,7 +3,7 @@
   "schema_version": 1,
   "package_version": "1.5.1",
   "totals": {
-    "tools": 206,
+    "tools": 209,
     "resources": 8,
     "prompts": 5,
     "groups": 23
@@ -15,7 +15,7 @@
     "corner_ops": 4,
     "dimensions": 5,
     "drawing": 11,
-    "engineering": 12,
+    "engineering": 15,
     "entity_creation": 18,
     "entity_modification": 16,
     "entity_query": 9,
@@ -343,6 +343,20 @@
         "destructiveHint": true
       }
     },
+    {
+      "name": "centre_marks",
+      "group": "engineering",
+      "cost": "mutate",
+      "summary": "Draw ISO 128-23 centre marks or centre lines on circles, read from the drawing.",
+      "tags": [
+        "engineering",
+        "mech"
+      ],
+      "annotations": {
+        "title": "Centre Marks (ISO 128-23)",
+        "destructiveHint": false
+      }
+    },
     {
       "name": "construction_clear",
       "group": "premium",
@@ -1592,6 +1606,20 @@
         "destructiveHint": false
       }
     },
+    {
+      "name": "hatch_material",
+      "group": "engineering",
+      "cost": "mutate",
+      "summary": "Hatch a region with the ISO 128-50 section pattern for a named material.",
+      "tags": [
+        "engineering",
+        "mech"
+      ],
+      "annotations": {
+        "title": "Material Hatch (ISO 128-50)",
+        "destructiveHint": false
+      }
+    },
     {
       "name": "hatch_set_gradient",
       "group": "entity_creation",
@@ -2212,6 +2240,20 @@
         "readOnlyHint": true
       }
     },
+    {
+      "name": "section_line",
+      "group": "engineering",
+      "cost": "mutate",
+      "summary": "Draw an ISO 128-40 cutting-plane line and return the plane the section view uses.",
+      "tags": [
+        "engineering",
+        "mech"
+      ],
+      "annotations": {
+        "title": "Section Line (ISO 128-40)",
+        "destructiveHint": false
+      }
+    },
     {
       "name": "selection_filter",
       "group": "entity_query",