feat: bundle dot-watch helper for live graphviz .dot -> PNG re-render in Studio

pi-studio renders Mermaid natively but has no DOT renderer. Its markdown
preview displays local PNG/JPG/GIF/WEBP images, so dot-watch closes the
loop for Graphviz: edit .dot -> auto-render <name>.png -> Studio
refresh-from-disk shows the update. Uses mtime polling (no inotify dep).

- rootfs/usr/local/bin/dot-watch: the helper (executable)
- Dockerfile.base: COPY + chmod, following the studio-expose pattern
- README.md: 'Graphviz diagrams in Studio' subsection
- CHANGELOG.md: Unreleased entry

graphviz was already in the base image; no new package.

README.md

@@ -276,6 +276,26 @@ Assuming the compose file publishes `127.0.0.1:8765:8765` (see method B):
 > which is **not** in `-studio` (only the planned `-studio-tex`). HTML
 > export, KaTeX, Mermaid, and all REPL features work without it.
 
+### Graphviz diagrams in Studio: `dot-watch`
+
+pi-studio renders **Mermaid** natively but has **no Graphviz/DOT renderer**.
+Its markdown preview *does* render local image links (`.png`/`.jpg`/`.gif`/
+`.webp`), so the workflow for Graphviz is: write a `.dot` file, render it to
+PNG with `dot`, and preview the PNG (directly, or embedded in a markdown
+file). The bundled **`dot-watch`** helper automates the re-render so edits
+show up on Studio's *refresh-from-disk*:
+
+```bash
+dot-watch graph.dot              # dot engine, 150 dpi -> graph.png
+dot-watch graph.dot neato 200    # pick layout engine + dpi
+```
+
+It polls the file's mtime (no `inotify` dependency) and regenerates
+`<name>.png` on every save, printing timestamped status and indenting any
+DOT syntax errors instead of crashing. Then in Studio: open the PNG (or a
+`.md` that embeds it) and hit **refresh-from-disk** after each edit.
+Note: SVG is **not** in Studio's local-image-link allowlist — use PNG.
+
 ## docker-compose.yml — basic shape
 
 ```yaml