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

engineering/understand/__init__.py

@@ -0,0 +1,57 @@
+"""Understanding drawings somebody else made (v1.6 track H).
+
+One bulk read per drawing (:mod:`.snapshot`), multilingual vocabulary
+(:mod:`.vocab`) and label parsers (:mod:`.labels`) are the foundation; the
+readers built on them - the one-call report, the scale check, the line
+networks and takeoffs, the diff and the topology checks - add their modules
+here. Everything except the snapshot's live export is pure: records in,
+results out, the drawing never modified.
+"""
+
+from engineering.understand.labels import (
+    normalize_panel,
+    parse_diameters,
+    parse_electrical,
+    parse_tag,
+    plain,
+    wiring_target,
+)
+from engineering.understand.snapshot import (
+    EntityRecord,
+    Pt,
+    Snapshot,
+    length_of,
+    read_snapshot,
+    records_from_doc,
+    take_snapshot,
+)
+from engineering.understand.vocab import (
+    DISCIPLINES,
+    SERVICES,
+    classify_layer,
+    equipment_kind,
+    room_label,
+    supply_return,
+)
+
+__all__ = [
+    "DISCIPLINES",
+    "SERVICES",
+    "EntityRecord",
+    "Pt",
+    "Snapshot",
+    "classify_layer",
+    "equipment_kind",
+    "length_of",
+    "normalize_panel",
+    "parse_diameters",
+    "parse_electrical",
+    "parse_tag",
+    "plain",
+    "read_snapshot",
+    "records_from_doc",
+    "room_label",
+    "supply_return",
+    "take_snapshot",
+    "wiring_target",
+]