feat(task-7): SECTION 21 - the mechanical part model as six tools

engineering/mech/draw.py is the only module in the package that touches a
backend: it turns the pure engine's primitive descriptions into entity_create_*
and dimension_* calls, places them in WCS, and writes the part model back onto
the drawing as ACADMCP_MECH XDATA on an anchor POINT. It is not a contract
member and neither are the tools - exactly like gear_draw_* and
titleblock_apply_iso_a3, it composes the generic members, so it is written once
and works on both engines by construction and no capability key is needed. No
new ActiveX call was written, so there was nothing to probe on the live seat;
measured instead that every one of the 23 backend members draw.py calls exists
on ComBackend and that ComBackend has no abstract methods left.

mech_part_draw takes one model - a turned profile or a plate outline plus typed
features - and draws its views in first-angle positions (third on request),
dimensions them and records the placement. mech_view_add reads the model back
out of the drawing and adds a side, top, section (full/half/offset/revolved) or
detail view months later without the caller re-describing the part.
mech_dimension_part lays out ISO 129 dimensions with no overlapping text,
resolves ISO 286 fits against the measured nominal, and moves the holes into a
table past a threshold. mech_hole_pattern exposes the same expansion for a
drawing that was never built from the model - with its own diameter guard,
because Hole.validate needs a part and a standalone pattern has none.
mech_part_inspect reads the payload back and never modifies the drawing.

mech_part_from_spec draws a whole sheet inside one transaction. Every part is
validated before the transaction opens, so a refusal leaves the drawing
untouched and names the path (parts[1]: segments[0].d_outer: ...), and the
rollback is shielded with the shape engineering/pid/spec.py documents: anyio
cancellation is level-triggered, so a bare await in the handler would be
cancelled before the rollback body ran and leave half a sheet inside an open
transaction.

fits goes through dimension_intents(fits=...), the API Task 6 shipped, rather
than being stamped on at draw time: that is the path that validates a key by
name and refuses a fit on a reference dimension or on a hole that moved into
the table. It is scoped per view first - a fit on segment[0] names nothing in
the end view, and refusing there would make a correct three-view sheet
undrawable - and any key no view claimed is handed back to dimension_intents so
the refusal is written in its own vocabulary.

SECTION 21 registers the six tools in the new mech pack, with alias records
(SECTIONPLANE, ARRAY plus shop-floor synonyms in English and Turkish - mil,
kademeli mil, kama, pah, kesit, gorunus), a regenerated tool inventory, the
README release snapshot at 210 tools in 24 groups, and the frozen group-size
snapshot gaining mech: 6. Lean is deliberately untouched here: spec section 11
puts three of these on the lean surface together with group G's two sheet tools,
and Task 15 makes that edit once across all four branches.

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

tests/test_tool_packs.py

@@ -86,7 +86,16 @@ LEAN_SETTINGS_ESSENTIALS = {
     "textstyle_set_current",
 }
 
-ALL_PACKS = ["core", "pid", "settings"]
+ALL_PACKS = ["core", "pid", "settings", "mech"]
+ALL_PACKS_SORTED = sorted(ALL_PACKS)
+MECH_TOOLS = {
+    "mech_part_draw",
+    "mech_view_add",
+    "mech_dimension_part",
+    "mech_hole_pattern",
+    "mech_part_from_spec",
+    "mech_part_inspect",
+}
 
 
 @pytest_asyncio.fixture(autouse=True)
@@ -97,8 +106,9 @@ async def _restore(monkeypatch):
 
 
 def test_pack_registry_names_real_tools_and_only_them():
-    assert server.TOOL_PACK_NAMES == ("core", "pid", "settings")
+    assert server.TOOL_PACK_NAMES == ("core", "pid", "settings", "mech")
     assert server.PACK_TOOL_NAMES["pid"] == frozenset(PID_TOOLS)
+    assert server.PACK_TOOL_NAMES["mech"] == frozenset(MECH_TOOLS)
     assert server.PACK_TOOL_NAMES["settings"] == frozenset(SETTINGS_TOOLS)
     assert len(SETTINGS_TOOLS) == 22
     assert not (server.PACK_TOOL_NAMES["settings"] & server.PACK_TOOL_NAMES["pid"])
@@ -137,7 +147,7 @@ async def test_all_is_the_default_and_enables_every_pack(monkeypatch):
     info = await server._apply_tool_profile("full")
     disabled = set(info["disabled_tools"])
     assert not ((PID_TOOLS | SETTINGS_TOOLS) & disabled)
-    assert info["tool_packs"]["enabled"] == ALL_PACKS
+    assert info["tool_packs"]["enabled"] == ALL_PACKS_SORTED
 
 
 async def test_unknown_pack_is_ignored_with_a_warning_and_core_cannot_be_dropped(
@@ -184,7 +194,8 @@ async def test_the_three_track_e_sections_file_under_their_own_groups():
     assert set(groups["styles"]) == STYLE_TOOLS
     assert set(groups["page_setup"]) == PAGE_SETUP_TOOLS
     assert set(groups["environment"]) == SETTINGS_TOOLS
-    assert len(groups) == 23
+    assert set(groups["mech"]) == MECH_TOOLS
+    assert len(groups) == 24
 
 
 async def test_system_about_reports_packs():