docs(mermaid): enable native Mermaid.js rendering via superfences fence

```mermaid fences (README architecture/update-flow/directory diagrams) were
emitted as plain code blocks because mkdocs.yml had no Mermaid integration.
Add the pymdownx.superfences custom fence (class=mermaid) documented by
Material for MkDocs — the theme then lazy-loads mermaid@11 and renders the
diagrams client-side, theme-aware (light/dark). Note: rendering needs
internet at view time (theme loads mermaid from unpkg CDN); without it the
raw source shows as fallback.

Also pin mkdocs-material==9.7.7 in the publish workflow and DOCS.md
(reproducible builds; 9.7.7 is the latest release, project is in
maintenance mode until 2026-11-05).

Author: localpibox

.github/workflows/docs-publish.yml

@@ -76,7 +76,7 @@ jobs:
           fi
 
       - name: Install docs tooling
-        run: python3 -m pip install --quiet mkdocs-material mike
+        run: python3 -m pip install --quiet "mkdocs-material==9.7.7" mike
 
       - name: Prepare docs tree + version stamp
         run: python3 scripts/generate.py ${TAG:+--tag "$TAG"}

DOCS.md

@@ -9,6 +9,10 @@ This branch carries the documentation site for the LocalPibox stack
 - **`docs/` is derived** (gitignored) — `scripts/generate.py` copies the
   tracked content and stamps version pages (repo map, versions) from the
   6 stack repos. Run it before every build; CI does this automatically.
+- **Mermaid diagrams** (` ```mermaid ` fences, e.g. in `README.md`) are
+  rendered client-side by the Material theme, which lazy-loads mermaid@11
+  from the unpkg CDN on first visit — viewing diagrams needs internet
+  access; without it the raw source shows as a code block (fallback).
 - **Versions** are cut by `.github/workflows/docs-publish.yml`:
   - push to `docs` → living `edge` version (always newest content)
   - dispatch with `tag: 0.0.X-lpb[-dev]` → immutable per-tag version + alias
@@ -19,7 +23,7 @@ This branch carries the documentation site for the LocalPibox stack
 ## Local build
 
 ```bash
-python3 -m pip install --user mkdocs-material mike
+python3 -m pip install --user "mkdocs-material==9.7.7" mike
 python3 scripts/generate.py          # or: --tag 0.0.53-lpb-dev
 mike serve                           # live preview at http://localhost:8000
 ```

mkdocs.yml

@@ -50,6 +50,13 @@ markdown_extensions:
   - admonition
   - attr_list
   - md_in_html
+  # Mermaid.js diagrams: theme renders ```mermaid fences client-side
+  # (lazy-loads mermaid@11 from CDN when a .mermaid block is present)
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
 
 nav:
   - Home: index.md