docs(readme): stop upscaling the hero sheet 17%

`view_screenshot` renders at 100 DPI with a tight bounding box, so the
showcase PNG is intrinsically 698 x 500. The README displayed it at
width="820" -- a ~17% browser upscale of a raster, on a page whose subject
is drafting precision, and next to four charts that are resolution-
independent SVG. Native width instead; sharp beats big.

The rest of the hero caption was re-audited against the DXF it describes
and every item holds: 7 engineering layers, involute gear outline, DIN 6885
keyed bore, hatched section A-A, and two ISO 129 dimensions now measuring
144.000 and 40.000 with the H7 callout -- the two that were wrong before the
diameter fix in 8f405bd.

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

README.md

@@ -21,7 +21,7 @@ Live through COM on Windows, or headless through ezdxf on any platform — one t
 
 <div align="center">
 
-<img src="https://raw.githubusercontent.com/U-C4N/Autocad-MCP/main/docs/assets/autocad-mcp-showcase.png" alt="ISO A3 sheet: a module-6, 24-tooth spur gear with a keyed bore, its section A-A, ISO 129 dimensions with an H7 bore fit, and an ISO 7200 title block" width="820">
+<img src="https://raw.githubusercontent.com/U-C4N/Autocad-MCP/main/docs/assets/autocad-mcp-showcase.png" alt="ISO A3 sheet: a module-6, 24-tooth spur gear with a keyed bore, its section A-A, ISO 129 dimensions with an H7 bore fit, and an ISO 7200 title block" width="698">
 
 <sub><b>Not a mockup.</b> Every line on this sheet was drawn by the tools this server exposes — ISO layers, involute gear geometry, DIN 6885 keyway, section A-A, ISO 129 dimensions, an ISO 286 <code>H7</code> bore fit, ISO 7200 title block — then rendered headlessly by <code>view_screenshot</code>. <code>drawing_critique</code> returns <b>0 issues</b> on it. Rebuild it with <code>python scripts/render_readme_showcase.py</code>.</sub>