Class-A surfacing 3/3: blend curves, curved-edge fillets, the G2 corner and continuity analysis (#1642)

Thanks @midasdf.

ATTRIBUTION.md

@@ -36,6 +36,8 @@ Editable drawing code, native projects and shipped exports are collected under `
 | assets/archive/icons/macos-v1/exports/icon.iconset/icon_256x256@2x.png | SolidCraft contributors | native sips export from assets/archive/icons/macos-v1/exports/icon.png | MIT OR Apache-2.0 |
 | assets/archive/icons/macos-v1/exports/icon.iconset/icon_512x512.png | SolidCraft contributors | native sips export from assets/archive/icons/macos-v1/exports/icon.png | MIT OR Apache-2.0 |
 | assets/archive/icons/macos-v1/exports/icon.iconset/icon_512x512@2x.png | SolidCraft contributors | native sips export from assets/archive/icons/macos-v1/exports/icon.png | MIT OR Apache-2.0 |
+| docs/screenshots/class-a-blend.png | SolidCraft contributors | screenshot of SolidCraft itself: two boxes bridged by `surface.blend` (a 40x10 sheet, `solidcraft-cli snapshot` of plan/evidence script ev_blend) | MIT OR Apache-2.0 |
+| docs/screenshots/class-a-crown.png | SolidCraft contributors | screenshot of SolidCraft itself: a patch crowned with `surface.patch crown=0.3` over a saddle rim (`solidcraft-cli snapshot`) | MIT OR Apache-2.0 |
 | docs/screenshots/class-a-boundary.png | SolidCraft contributors | screenshot of SolidCraft itself: a patch through two sketch lines built by `surface.boundary` with `crown: 0.4` (`solidcraft-cli snapshot`) | MIT OR Apache-2.0 |
 | docs/screenshots/sample-plate.png | SolidCraft contributors | screenshot of SolidCraft itself (`solidcraft --sample`) | MIT OR Apache-2.0 |
 | docs/screenshots/demo-0.8.gif | SolidCraft contributors | original screen recording of the actual SolidCraft 0.8 English desktop UI modelling an original Raspberry Pi 4 case (no logos or branding; the engraved text is set in the system's DejaVu Sans Bold), generated with examples/demo/record_ui.sh and its original animated pointer overlay (9× playback) | MIT OR Apache-2.0 |