feat(understand): the snapshot, the vocabulary and the label parsers; MAX_DXF_BYTES 512 MB; the office extra

Track H's foundation, landed on main before its three groups branch.

- engineering/understand/snapshot.py: one bulk read of a drawing into frozen
  WCS records (OCS translated through backends/ocs.py, text decoded, an
  INSERT boxed by its block geometry without its attribute texts). A DXF is
  read with ezdxf after the size check; the headless engine's document is
  read under its own lock; the live engine exports the active document with
  Document.Export(<tmp>/snap, "DXF", <set of all>) - never renamed, deleted
  afterwards - and a DWG is opened read-only, exported and closed. Measured on
  AutoCAD 2026: ".dxf" is refused with Invalid argument, the file lands as
  snap.DXF, model space, paper space and unused block definitions all reach
  it, and the selection set does not narrow a DXF export.
- engineering/understand/labels.py: %%c, \U+2205, the U+2300 sign, o-stroke
  and MTEXT format runs all read as one diameter spelling; sizes are kept as
  drawn and never equated; Cyrillic look-alike capitals fold into Latin tags;
  panels, wiring callouts and electrical data (kW, phases, volts, N, VFD) in
  EN / TR / RU / NL / DE. A power that is not written stays None.
- engineering/understand/vocab.py: layer classification with a confidence and
  the deciding keyword, supply / return words, room labels and block kinds in
  five languages. A CIP layer without a direction word is not guessed.
- MAX_DXF_BYTES defaults to 512 MB (a real layout was 188 MB); the refusal
  names the size and the variable. New `office` extra (openpyxl), also in
  `full`; uv.lock regenerated.

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

pyproject.toml

@@ -88,6 +88,12 @@ pdf = [
     # Also satisfies ezdxf's hard Pillow requirement, transitively.
     "matplotlib>=3.8",
 ]
+office = [
+    # XLSX output: the takeoff workbooks (pipe_takeoff / cable_takeoff) and
+    # data_extract(format="xlsx"). Without it every one of them still writes
+    # CSV and refuses XLSX with the `xlsx_write` capability, naming this extra.
+    "openpyxl>=3.1",
+]
 full = [
     # Same marker as `com` above, and for the same reason: `[full]` is the
     # extra this project recommends to contributors, and it was
@@ -95,6 +101,7 @@ full = [
     'pywin32>=306; sys_platform == "win32"',
     "Pillow>=10.0",
     "matplotlib>=3.8",
+    "openpyxl>=3.1",
 ]
 
 [project.scripts]