feat(mech): SECTION 23 opens with surface_texture and weld_symbol

ISO 1302:2002's seven transcribed symbol-proportion rows (text height ->
line width d', short-leg height H1, minimum height H2) drive the 60 deg
vee, the machining-required bar at H1, the machining-prohibited circle
inscribed in the vee, and the all-around circle at the kink; the Ra/Rz
values, the process, the lay symbol and the machining allowance land
around the extension line, which grows with the widest of them. A text
height outside the seven rows is refused by name rather than
interpolated, and ISO 1302 is accepted as an alias designation for
drawings still issued under it - the graphical symbol is the same one.

ISO 2553:2019's reference line, dashed identification line (omitted for a
symmetrical weld, as the standard permits), size written before the
symbol, length and pitch written after it, field-weld flag, all-around
circle and process tail. Six elementary symbols are transcribed - square,
single-V, single-bevel, single-U, single-J, fillet. The rest of the
elementary table is deliberately not shipped: a weld kind outside the six
is refused with the list of the six, because a guessed weld symbol is a
wrong instruction to a welder.

Both tools place the symbol on a plain leader - a line plus an arrowhead
with the proportions the backend's own mleader already draws - rather
than through leader_create_mleader, which refuses empty text and would
hang an MTEXT label off every symbol.

Proportions neither standard dimensions (the all-around circles, the
flag, the identification line's dashes, the text pitch and the character
width estimate) are named constants documented as this implementation's
declared choices, never as transcribed values.

No new ActiveX member is called: both tools compose LINE / ARC / CIRCLE /
LWPOLYLINE / TEXT through the entity-creation contracts both engines
already implement, so no live COM probe was needed.

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

tests/test_tool_registry.py

@@ -495,7 +495,7 @@ async def test_tool_groups_is_byte_identical_to_the_source_declared_grouping():
 
 
 async def test_tool_group_sizes_are_unchanged():
-    """Frozen snapshot of the surface (204 tools, 23 groups).
+    """Frozen snapshot of the surface (206 tools, 23 groups).
 
     Taken before @cad_tool landed at 131 tools; `batch` moved 2 -> 3 when
     v1.5.0's `cad_batch` joined `entity_batch_create`/`entity_batch_modify`, and
@@ -529,6 +529,11 @@ async def test_tool_group_sizes_are_unchanged():
     their secondary tags; those counts returned to their pre-track-E values
     when the section tags landed.) Every other number here has been unchanged
     since the snapshot was taken.
+
+    `engineering` moved 10 -> 12 when v1.6's SECTION 23 opened with
+    `surface_texture` / `weld_symbol`; both are tagged `engineering` and `mech`,
+    and `mech` is not in `_GROUP_TAG_PRIORITY`, so they file under
+    `engineering` rather than opening a group of their own.
     """
     sizes = {label: len(names) for label, names in (await server._tool_groups()).items()}
     assert sizes == {
@@ -538,7 +543,7 @@ async def test_tool_group_sizes_are_unchanged():
         "corner_ops": 4,
         "dimensions": 5,
         "drawing": 11,
-        "engineering": 10,
+        "engineering": 12,
         "entity_creation": 18,
         "entity_modification": 16,
         "entity_query": 9,
@@ -556,4 +561,4 @@ async def test_tool_group_sizes_are_unchanged():
         "validation": 1,
         "view": 4,
     }
-    assert sum(sizes.values()) == 204
+    assert sum(sizes.values()) == 206