feat(understand): drawing_understand - units, extents, clusters, layers, tags, rooms

A drawing somebody else made, read back as one report in which every
claim carries its evidence and a confidence; the drawing is never
modified.

- units.py: $INSUNITS against the unit the geometry implies (text
  heights, wall thickness, door swings, labelled room areas, overall
  size). The header is overruled only when the evidence outweighs it,
  and then with a warning that quotes the numbers. Robust extents grow
  from the quartile box of entity centres; what never joins is an
  outlier, named by handle.
- clusters.py: separate bodies of model-space content by raster,
  8-connectivity and box absorption - two copies of one plan are two
  clusters.
- describe.py: layers with drawn length and the vocabulary's meaning
  (name keyword, else the texts on the layer), equipment tags (one tag
  lettered twice beside its symbol merged, repeats flagged ambiguous),
  room labels in the wall-layer faces of engineering.arch.faces, text
  languages by character class, blocks, layouts, the title block.
- server.py opens SECTION 26 (Understanding & QA) with
  drawing_understand: core, read-only, tagged analysis/query. Alias
  record, inventory, README release snapshot 242, frozen group sizes
  (analysis 13).

Co-Authored-By: Claude Opus 5.5 (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 (241 tools, 26 groups).
+    """Frozen snapshot of the surface (242 tools, 26 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
@@ -591,10 +591,17 @@ async def test_tool_group_sizes_are_unchanged():
     exactly `PACK_TOOL_NAMES["arch"]`, and every group the track-F branches
     had grown while the tag was unranked on them returned to its
     pre-track-F value.
+
+    Track H group U opened SECTION 26 (Understanding & QA) with
+    `drawing_understand`, tagged `analysis` and `query`: `analysis` outranks
+    `query` in `_GROUP_TAG_PRIORITY`, so the reader files under `analysis`
+    (12 -> 13) beside the other whole-drawing queries. This snapshot is
+    branch-local: the track H merge task recomputes it once the understand,
+    network and diff branches have landed.
     """
     sizes = {label: len(names) for label, names in (await server._tool_groups()).items()}
     assert sizes == {
-        "analysis": 12,
+        "analysis": 13,
         "architecture": 12,
         "batch": 3,
         "blocks": 9,
@@ -621,4 +628,4 @@ async def test_tool_group_sizes_are_unchanged():
         "validation": 1,
         "view": 4,
     }
-    assert sum(sizes.values()) == 241
+    assert sum(sizes.values()) == 242