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

server.py

@@ -10554,6 +10554,80 @@ async def arch_plan_from_spec(
         raise ToolError(str(exc)) from exc
 
 
+# ---------------------------------------------------------------------------
+# ── SECTION 26: Understanding & QA (1 tool) ─────────────────────────────────
+# ---------------------------------------------------------------------------
+
+
+@cad_tool(
+    summary="Read a foreign drawing: units, extents, clusters, layers, tags, rooms, languages.",
+    cost="read",
+)
+@mcp.tool(
+    annotations={"title": "Understand: Describe Drawing", "readOnlyHint": True},
+    tags={"analysis", "query"},
+)
+async def drawing_understand(
+    path: Annotated[
+        str | None,
+        Field(
+            default=None,
+            description="DXF file to read (a DWG only on a live seat); omitted: the current "
+            "document.",
+        ),
+    ] = None,
+    ctx: Context = None,
+) -> dict:
+    """One report on a drawing somebody else made, every claim with its evidence
+    and a confidence. Never modifies the drawing: a file is read as it is, and
+    the current document through a snapshot (headless: the document in memory;
+    live: a `Document.Export` DXF copy in a temporary folder, deleted after).
+    An xref's own entities are not read - the host drawing is.
+
+    - `units`: `$INSUNITS` as declared against the unit the geometry implies
+      (text heights, wall thickness, door swings, labelled room areas, overall
+      size - each item with its median and its value under every candidate
+      unit). A disagreement is a warning with the numbers, never a silent
+      correction.
+    - `extents`: declared, computed and robust boxes and the `outliers` that
+      stretch them, farthest first, by handle.
+    - `clusters`: separate bodies of model-space content (plan copies, sheets,
+      details), each with its box, entity count, commonest layers and tallest
+      labels.
+    - `layers`: entity count, drawn length (and metres in the inferred unit)
+      and the discipline / service each most likely carries - the vocabulary's
+      confidence (0.9) for a name keyword, 0.6 when most texts on the layer
+      agree, 0.4 when fewer do.
+    - `equipment_tags` (a tag found more than once is `ambiguous`), `rooms`
+      (room labels in five languages, each with the wall-layer face it sits in
+      and its measured area), `languages` (Latin / Turkish / Cyrillic character
+      classes), `blocks` (named / anonymous, kinds by name), `layouts`,
+      `title_block` and `warnings`.
+
+    Refused before anything is read: a `path` that does not exist, a DXF over
+    `MAX_DXF_BYTES` (the refusal names the size and the variable), and a file
+    the engine cannot parse (a DWG headlessly: ezdxf reads DXF only).
+    """
+    import asyncio
+
+    from ezdxf.lldxf.const import DXFError
+
+    from engineering.understand.describe import describe
+    from engineering.understand.snapshot import take_snapshot
+
+    target = None
+    if path:
+        target = validate_path(path)
+        if not target.is_file():
+            raise ToolError(f"path: no such file {str(target)!r}")
+    await ctx.info(f"drawing_understand: {target or 'current document'}")
+    try:
+        snap = await take_snapshot(_backend(ctx), str(target) if target else None)
+    except (ValueError, OSError, RuntimeError, DXFError) as exc:
+        raise ToolError(str(exc)) from exc
+    return await asyncio.to_thread(describe, snap)
+
+
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 # ── RESOURCES ───────────────────────────────────────────────────────────────
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