bench: matrix v5 adds mech_assembly; the correctness suite grows with the mechanical and sheet checks

The evidence for tracks B and G, in the shape tracks A and E used.

- benchmarks/tasks_v5.py: 18 tasks, DEFAULT_MATRIX = "v5"; v4, v3 and v2 stay
  addressable so an earlier report reproduces exactly rather than being only
  described. mech_assembly is roadmap criterion 3 - a flange-coupling sheet
  built from the part model and the sheet standard, through the code the
  tools call (apply_titleblock, draw_part/add_view/dimension_part,
  insert_std_part, extract_records -> draw_bom_table, add_balloon), gated on
  two numbers that cannot be negotiated: drawing_critique(focus=None) returns
  zero issues and the finalize score is at least 90 (it scores 100, 18/18 on
  the v5 matrix). The adapter returns the focuses that fired, so a
  regression names itself, and a test drops one balloon to prove the gate
  fails and names mech_bom_balloon_mismatch.
- Seven correctness checks (32 -> 39). mech_section_hatch_area is the one
  that cannot be faked: a sleeve of length 60, outside diameter 40 and bore
  20 cuts two faces totalling 1200 mm^2 exactly, read back out of the drawing
  through entity_measure. sheet_frame_iso5457_a3 pins the A3 frame (20,10 to
  410,287 inside the 420 x 297 trimmed sheet); mech_iso286_on_dimension pins
  40 H7 at +0.025/0 through the layout; std_part_iso4014_m12 pins s = 18 and
  k = 7.5 by value; mech_thread_unrepresented_is_caught is the seventh: a
  quality gate that never fires is not a gate.
- scripts/smoke_mech_com.py executed every new COM path once on AutoCAD 2026
  (25.1s): frame, title block, a sectioned shaft with 12 dimensions, an ISO
  4014 block with 1 ACADMCP_MECH chunk, a weld symbol, the parts list read off
  the drawing, a linked balloon, an xref attached / listed / detached, a
  31687-byte .dwg (AC1032) through ActiveX SaveAs, critique 0 issues, exit 0,
  every scratch document closed. It retries RPC_E_CALL_REJECTED (measured:
  the first call after Documents.Add is refused) and tracks its documents by
  the document list, since SaveAs renames them.
- Defects the evidence found, each fixed with a test that fails on the old
  code:
  * ezdxf dimension_diameter put the text beyond the SECOND chord point;
    ActiveX AddDimDiametric puts it beyond the first (measured live: a d50
    from (x,170) to (x,220), leader 10, text at (x,160)). Headlessly every
    front-view diameter stood on its own chain dimension. ezdxf now follows
    ActiveX.
  * draw_part's anchor goes on MECH; ActiveX refuses entity.Layer on a
    missing layer (measured -0x7ffdfff7 'Key not found'). draw.py ensures
    every target layer before the first entity.
  * bom.extract_records filtered on INSERT; ActiveX names it BLOCKREFERENCE
    (ObjectName AcDbBlockReference) - the live parts list was empty.
  * COM _entity_info reported no dimension text_position: ModelSpace.Item
    narrows to IAcadEntity, which has no TextPosition (measured
    AttributeError); read through _com_unnarrow (TextPosition measured on
    AcDbDiametricDimension, WCS). dim_overlap compared bbox centres and a
    diameter collided with its own bore.
  * pid graph raised KeyError 'start' mid-critique on a LINE whose point read
    AutoCAD refused while busy; such a line is left out.
  * the validator read "ISO 4014 - M12x60" as fake dimension text.
- The published A/B report is re-run over the grown suite rather than
  re-typed (26/39 -> 39/39, 13 fixed, 0 regressed), check_doc_numbers derives
  the task count from the v5 matrix, the token table is remeasured with a
  TOOL_PACKS=core,mech row (193 tools / 61,673 tokens), and the README's
  collected-tests figure is retaken (3990).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

engineering/mech/draw.py

@@ -109,6 +109,7 @@ async def draw_prims(
     """Draw primitive descriptions, turned by ``rotation`` about their origin, then placed at ``at``."""
     placed = tuple(prims)
     _refuse_islands_without_edge_paths(backend, placed)
+    await _ensure_layers(backend, sorted({f"{layer_prefix}{layer_for(p)}" for p in placed}))
     if abs(float(rotation)) > 1e-12:
         placed = rotate(placed, float(rotation), (0.0, 0.0))
     if abs(float(at[0])) > 1e-12 or abs(float(at[1])) > 1e-12:
@@ -195,6 +196,23 @@ async def draw_prims(
     return {"handles": handles, "counts": counts}
 
 
+async def _ensure_layers(backend, names) -> tuple[str, ...]:
+    """Create whichever target layers the drawing lacks, before the first entity.
+
+    MEASURED on AutoCAD 2026 by ``scripts/smoke_mech_com.py``: ``draw_part``
+    put its anchor POINT on ``MECH``, a layer no layer set creates, and
+    ActiveX answered ``entity.Layer = "MECH"`` with ``-0x7ffdfff7 ('Key not
+    found')`` - after every view had already been drawn. The headless engine
+    creates a missing layer on assignment, so the same call passed every test.
+    Group A measured the same refusal for annotation layers; this is the same
+    cure (``ensure_annotation_layers``) at the places every mechanical drawing
+    path passes through.
+    """
+    from engineering.mech.annotate import ensure_annotation_layers
+
+    return await ensure_annotation_layers(backend, [str(name) for name in names if name])
+
+
 def _refuse_islands_without_edge_paths(backend, prims) -> None:
     """Refuse a hatch with islands on an engine that cannot attach them - before drawing.
 
@@ -515,6 +533,7 @@ async def draw_part(
         if index == 0:
             parent, parent_offset = view, offset
 
+    await _ensure_layers(backend, [ANCHOR_LAYER])
     anchor = await backend.entity_create_point(origin[0], origin[1], layer=ANCHOR_LAYER)
     payload = {
         "v": PAYLOAD_VERSION,
@@ -839,6 +858,7 @@ async def dimension_part(
     skipped: list[dict] = []
     table_handle: str | None = None
 
+    await _ensure_layers(backend, [ROLE_LAYER["dim"]])
     for index, view, placed in planned:
         record = records[index]
         offset = record["offset"]
@@ -963,6 +983,7 @@ async def draw_hole_pattern(
             prims.append(Line((hole.x - reach, hole.y), (hole.x + reach, hole.y), "center"))
             prims.append(Line((hole.x, hole.y - reach), (hole.x, hole.y + reach), "center"))
     drawn = await draw_prims(backend, prims)
+    await _ensure_layers(backend, [layer, ROLE_LAYER["dim"] if dimension else None])
     if layer:
         # the role map owns the layers; an explicit override moves the circles
         # only, so the ISO 128-23 centre marks stay on CENTER.