fix(dimensions): diameter and radius callouts measured the leader as geometry

`add_diameter_dim` / `add_radius_dim` measure to the point they are handed;
`leader_length` is a text placement. The ezdxf backend passed
`centre + (radius + leader)` as that point, so the leader became part of the
measurement: a true 40 mm bore was dimensioned 60 at the default
`leader_length=10`, and 108 at 34. Radius callouts were `leader_length` too
large the same way.

This is the worst class of defect for this project -- a wrong number, at
default settings, on the one part of a drawing that exists to carry numbers --
and it shipped in v1.4.0 and earlier. The size now comes from `radius` and
`location` only places the text.

The live COM backend was never affected: ActiveX's AddDimDiametric takes the
two chord points and the leader length as separate arguments. The two engines
had been disagreeing by 2 x leader_length on the same call.

Found while building the drawing for the new README, whose own bore callout was
wrong. Two checks added to correctness_suite.py; both report fail -> pass
against v1.4.0, not miss -> pass, which is what separates a repaired defect
from new capability in that lane. A/B is now 21/26 -> 26/26, five fixed, zero
regressed.

README rewritten from scratch
- Hero is a real ISO A3 sheet the server drew: m6 z24 involute spur gear, DIN
  6885 keyed bore, section A-A, ISO 129 dimensions, ISO 286 H7 fit, ISO 7200
  title block, 56 entities, drawing_critique clean. New
  scripts/render_readme_showcase.py rebuilds it, and the tool-call sequence
  that produces it is printed in the page.
- Star history chart, badge row, capability matrix, five evidence lanes, and a
  "what this release is bad at" section carrying the 28% throughput regression.
- Every image and repo link is now an absolute URL: README.md is the PyPI long
  description, where relative paths 404.

Corrections this work forced, all measured
- The df=0 example was wrong. FILLET is already in entity_fillet's own text;
  the commands that genuinely scored zero are WBLOCK, PEDIT, OVERKILL,
  LAYTRANS and MATCHPROP.
- 40,305 tokens is the tools/list wire payload; ~25,143 is the part forwarded
  to the model. The old wording said "of schema", which is neither.
- `--tokenizer anthropic` needs a package and credentials this repo does not
  lock, and refuses rather than falling back. Said so.
- Discovery ranks over the 149-tool advertised catalog, not the 154 registered.
- Five middlewares, not four; the fifth is what makes refusals machine-readable.
- Five of the 27 capability keys are environment-dependent, so the matrix
  carries footnotes instead of bare ticks.
- "Every coordinate is WCS" has one deliberate exception: TEXT rotation.
- render_live_chart's axis still said "fixed 10-task matrix" while our bar came
  from v3's fifteen. Each bar now carries its own matrix.
- .env.example said MAX_UNDO_STACK=50 (code: 5) and omitted four real
  variables, two of them the remote-HTTP controls.
- server.json described an empty ALLOWED_PATHS as blocking access. It disables
  the allowlist.
- server.py called the lean profile ~46 tools; it is 47.
- backends/capability.py cited a test file that does not exist.

1249 passed, 1 xfailed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backends/ezdxf_backend.py

@@ -2968,13 +2968,15 @@ class EzdxfBackend(AutoCADBackend):
             radius = math.sqrt((chord_x - cxf) ** 2 + (chord_y - cyf) ** 2)
             angle_rad = math.atan2(chord_y - cyf, chord_x - cxf)
             leader = float(leader_length)
-            mpoint = (
-                cxf + (radius + leader) * math.cos(angle_rad),
-                cyf + (radius + leader) * math.sin(angle_rad),
-            )
+            # Same correction as dimension_diameter: the size comes from
+            # `radius`, and `location` only places the text.
             dim = msp.add_radius_dim(
                 center=(cxf, cyf),
-                mpoint=mpoint,
+                radius=radius,
+                location=(
+                    cxf + (radius + leader) * math.cos(angle_rad),
+                    cyf + (radius + leader) * math.sin(angle_rad),
+                ),
                 text=text if text is not None else "<>",
                 override=override or None,
             )
@@ -3011,13 +3013,18 @@ class EzdxfBackend(AutoCADBackend):
             radius = math.sqrt((x2f - x1f) ** 2 + (y2f - y1f) ** 2) / 2
             angle_rad = math.atan2(y2f - y1f, x2f - x1f)
             leader = float(leader_length)
-            mpoint = (
-                cx + (radius + leader) * math.cos(angle_rad),
-                cy + (radius + leader) * math.sin(angle_rad),
-            )
+            # The measured size comes from `radius`; `location` only places the
+            # text. This used to pass centre + (radius + leader) as `mpoint`,
+            # which ezdxf measures to — so every diameter came out
+            # 2 x leader_length too large. 60 on a true 40, at default
+            # settings, with nothing on the drawing to show for it.
             dim = msp.add_diameter_dim(
                 center=(cx, cy),
-                mpoint=mpoint,
+                radius=radius,
+                location=(
+                    cx + (radius + leader) * math.cos(angle_rad),
+                    cy + (radius + leader) * math.sin(angle_rad),
+                ),
                 text=text if text is not None else "<>",
                 override=override or None,
             )