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>

benchmarks/correctness_suite.py

@@ -269,6 +269,30 @@ async def two_vertex_circle_has_area():
     return abs(area - math.pi * 100.0) < 1e-9
 
 
+async def diameter_dim_measures_the_diameter():
+    """The number on the callout is the whole point of the callout.
+
+    `add_diameter_dim` measures to `mpoint`; the leader is a text placement.
+    Feeding it centre + (radius + leader) made every diameter come back
+    2 x leader_length too large at DEFAULT settings - 60 on a true 40.
+    """
+    b = await _b()
+    info = await b.dimension_diameter(-20, 0, 20, 0)
+    dim = b._doc.entitydb.get(info.handle)
+    return abs(dim.get_measurement() - 40.0) < 1e-6
+
+
+async def radius_dim_ignores_the_leader_length():
+    b = await _b()
+    near = await b.dimension_radius(0, 0, 20, 0, leader_length=5)
+    far = await b.dimension_radius(0, 0, 20, 0, leader_length=40)
+    values = [
+        b._doc.entitydb.get(near.handle).get_measurement(),
+        b._doc.entitydb.get(far.handle).get_measurement(),
+    ]
+    return all(abs(v - 20.0) < 1e-6 for v in values)
+
+
 CHECKS = {
     "core_line_length": (core_line_length, "Core"),
     "core_circle_radius": (core_circle_radius, "Core"),
@@ -294,6 +318,8 @@ CHECKS = {
     "hatch_area_subtracts_its_island": (hatch_area_subtracts_its_island, "Measurement"),
     "boundary_area_agrees_with_measure": (boundary_area_agrees_with_measure, "Measurement"),
     "two_vertex_circle_has_area": (two_vertex_circle_has_area, "Measurement"),
+    "diameter_dim_measures_the_diameter": (diameter_dim_measures_the_diameter, "Dimensions"),
+    "radius_dim_ignores_the_leader_length": (radius_dim_ignores_the_leader_length, "Dimensions"),
 }