make: prevent svg/png generation when dot is unavailable (#117)

## Summary

This is a small follow up to review comments from PR #111 where adding a
fast-fail check for Graphviz (dot) was suggested but not included before
merge.

## Changes

- Add check-graphviz target

- Wire it into svg and png

- Add to .PHONY

- Provide OS agnostic error message

## Before

running:
```
make svg
```

produces many repeated shell errors after generating all .dot files:

```
Converting assert_eq.smir.dot -> assert_eq.smir.svg
/bin/sh: line 4: dot: command not found
...
make: *** [Makefile:94: svg] Error 127
```

## After

```
$ make svg
Error: Graphviz is not installed or 'dot' is not in PATH.
Please install Graphviz for your system and ensure 'dot' is available.
See: https://graphviz.org/download/
```

## How to test

1. Temporarily uninstall Graphviz or remove dot from PATH

2. Run: make svg

3. Observe immediate fast-fail message

4. Reinstall Graphviz

5. Run again. Graphs generate normally
## Context

Reviewer suggestion here:


#111 (comment)


#111 (comment)


#111 (comment)


#111 (comment)

Co-authored-by: Daniel Cumming <124537596+dkcumming@users.noreply.github.com>

Author: 0xh4ty

Makefile

@@ -70,7 +70,7 @@ test-ui:
 	fi
 	bash tests/ui/run_ui_tests.sh "$$RUST_DIR_ROOT" "${VERBOSE}"
 
-.PHONY: dot svg png d2 clean-graphs
+.PHONY: dot svg png d2 clean-graphs check-graphviz
 
 OUTDIR_DOT=output-dot
 OUTDIR_SVG=output-svg
@@ -80,6 +80,14 @@ OUTDIR_D2=output-d2
 clean-graphs:
 	@rm -rf $(OUTDIR_DOT) $(OUTDIR_SVG) $(OUTDIR_PNG) $(OUTDIR_D2)
 
+check-graphviz:
+	@command -v dot >/dev/null 2>&1 || { \
+		echo "Error: Graphviz is not installed or 'dot' is not in PATH."; \
+		echo "Please install Graphviz for your system and ensure 'dot' is available."; \
+		echo "See: https://graphviz.org/download/"; \
+		exit 1; \
+	}
+
 dot:
 	@mkdir -p $(OUTDIR_DOT)
 	@for rs in $(TESTDIR)/*.rs; do \
@@ -89,15 +97,15 @@ dot:
 		mv $$name.smir.dot $(OUTDIR_DOT)/ 2>/dev/null || true; \
 	done
 
-svg: dot
+svg: check-graphviz dot
 	@mkdir -p $(OUTDIR_SVG)
 	@for dotfile in $(OUTDIR_DOT)/*.dot; do \
 		name=$$(basename $$dotfile .dot); \
 		echo "Converting $$name.dot -> $$name.svg"; \
 		dot -Tsvg $$dotfile -o $(OUTDIR_SVG)/$$name.svg; \
 	done
 
-png: dot
+png: check-graphviz dot
 	@mkdir -p $(OUTDIR_PNG)
 	@for dotfile in $(OUTDIR_DOT)/*.dot; do \
 		name=$$(basename $$dotfile .dot); \