test: add mermaid pipeline test slide

docs/test-mermaid.md

@@ -0,0 +1,35 @@
+---
+marp: true
+theme: gaia
+class: invert
+paginate: true
+---
+
+# Mermaid Pipeline Test
+
+Validating mermaid diagram rendering in Marp slides.
+
+MakerFLOSS · May 2026
+
+---
+
+## Build Pipeline
+
+```mermaid
+graph LR
+    A[Write Markdown] --> B[git push]
+    B --> C[Forgejo webhook]
+    C --> D[build-slides.sh]
+    D --> E[mermaid inject]
+    E --> F[slides.makerfloss.eu]
+```
+
+---
+
+## Regular Slide
+
+No diagram here — confirms regular content is unaffected.
+
+- Bullet one
+- Bullet two
+- Bullet three