feat(understand): drawing_diff - two revisions matched by geometric signature

engineering/understand/diff.py reads each revision once into entity
records and matches them in three stages: exact signature (type, layer,
geometry rounded to tol, text, block, rotation, scale, attributes;
handles ignored), then handle while at least half of the exact matches
kept theirs, then type + layer + nearest anchor within move_tol (default
2 % of the 1st-99th percentile diagonal). The report counts changes by
layer and type, says what changed per entity (moved by, text, attribute,
radius ... from -> to), compares named block definitions by content and
groups the changes into clusters.

drawing_diff opens group D's copy of SECTION 26. It is read-only unless
markup=True, which files revision clouds around the model-space clusters
on the current document through add_revision - refused before any read
with new_path, without a valid rev, or on an engine without revcloud.
A revision path that is neither .dxf nor .dwg is refused naming the
parameter (old_path: / new_path:), like every other path refusal.

Tests: the synthetic plant P&ID edited five ways (a move, a text, an
attribute, an add, a delete) under renumbered handles reports exactly
those five edits; renumbering alone reports nothing.

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

tests/fixtures/revisions.py

@@ -0,0 +1,96 @@
+"""Two revisions of the synthetic plant P&ID with known edits, for ``drawing_diff``.
+
+Built from ``build_plant_pair`` (``tests/fixtures/plant_pair.py``) with
+ezdxf. The older revision is the P&ID plus one tagged block reference; the
+newer one moves a line by half the move tolerance, rewrites a text, changes
+the block's attribute, deletes one line, adds a circle - and then gives every
+model-space entity a new handle, the way a save through another program
+renumbers them. What was edited is
+picked from the older revision's own records (entities whose signature is
+unique, so a double-drawn line can never be the one edited) and returned with
+the paths: the expected answer is read off the fixture, never typed in.
+"""
+
+from __future__ import annotations
+
+from collections import Counter
+from pathlib import Path
+
+import ezdxf
+
+from engineering.understand.diff import diff_snapshots, read_revision, robust_box, signature
+from engineering.understand.snapshot import records_from_doc
+from tests.fixtures.plant_pair import build_plant_pair
+
+TAG_BLOCK = "DIFF_TAG"
+TAG_LAYER = "DIFF-TAGS"
+ADDED_LAYER = "DIFF-ADDED"
+NEW_TEXT = "REV-B NOTE"
+TAG_BEFORE = "X-1"
+TAG_AFTER = "X-2"
+
+
+def renumber_modelspace(doc) -> None:
+    """Every model-space entity replaced by a copy under a fresh handle."""
+    msp = doc.modelspace()
+    for entity in list(msp):
+        clone = entity.copy()
+        msp.add_entity(clone)
+        msp.delete_entity(entity)
+
+
+def build_revision_pair(directory, *, edits: bool = True) -> dict:
+    directory = Path(directory)
+    truth = build_plant_pair(directory / "plant")
+    base = ezdxf.readfile(truth["pid"])
+    box = robust_box(records_from_doc(base, source="base"))
+    centre = ((box[0] + box[2]) / 2.0, (box[1] + box[3]) / 2.0)
+    block = base.blocks.new(TAG_BLOCK)
+    block.add_circle((0.0, 0.0), 5.0)
+    block.add_attdef("TAG", (0.0, -8.0), dxfattribs={"height": 2.5})
+    base.layers.add(TAG_LAYER)
+    tag = base.modelspace().add_blockref(TAG_BLOCK, centre, dxfattribs={"layer": TAG_LAYER})
+    tag.add_auto_attribs({"TAG": TAG_BEFORE})
+    old_path = directory / "rev_a.dxf"
+    base.saveas(old_path)
+
+    old_snap, _blocks = read_revision(str(old_path))
+    move_tol = diff_snapshots(old_snap, old_snap)["move_tol"]
+    counts = Counter(signature(r) for r in old_snap.records)
+    unique = [r for r in old_snap.records if r.space == "Model" and counts[signature(r)] == 1]
+    moved = next(r for r in unique if r.type == "LINE")
+    text = next(r for r in unique if r.type in ("TEXT", "MTEXT"))
+    insert = next(r for r in old_snap.records if r.block == TAG_BLOCK)
+    deleted = next(
+        r
+        for r in unique
+        if r.type in ("LINE", "LWPOLYLINE") and (r.type, r.layer) != (moved.type, moved.layer)
+    )
+
+    new = ezdxf.readfile(old_path)
+    msp = new.modelspace()
+    if edits:
+        new.entitydb[moved.handle].translate(move_tol / 2.0, 0.0, 0.0)
+        target = new.entitydb[text.handle]
+        if target.dxftype() == "MTEXT":
+            target.text = NEW_TEXT
+        else:
+            target.dxf.text = NEW_TEXT
+        new.entitydb[insert.handle].get_attrib("TAG").dxf.text = TAG_AFTER
+        msp.delete_entity(new.entitydb[deleted.handle])
+        new.layers.add(ADDED_LAYER)
+        msp.add_circle(centre, move_tol / 10.0, dxfattribs={"layer": ADDED_LAYER})
+    renumber_modelspace(new)
+    new_path = directory / "rev_b.dxf"
+    new.saveas(new_path)
+    return {
+        "old": str(old_path),
+        "new": str(new_path),
+        "move_tol": move_tol,
+        "old_count": len(old_snap.records),
+        "moved": moved.handle,
+        "text": text.handle,
+        "text_before": text.text,
+        "insert": insert.handle,
+        "deleted": deleted.handle,
+    }