docs: disable playground run button

Rust Playground only supports top 100 crates, so code blocks
using problemreductions can't run there. Disable globally.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: GiggleLiu

book.toml

@@ -20,6 +20,7 @@ level = 1
 [output.html.playground]
 editable = false
 copy-js = true
+runnable = false
 
 [output.html.search]
 enable = true

docs/src/arch.md

@@ -33,7 +33,7 @@ MaximumIndependentSet/Weighted  # weighted objective
 
 Evaluating a configuration returns both validity and objective value:
 
-```rust,ignore
+```rust
 let config = vec![1, 0, 1, 0];  // Variable assignments
 let result = problem.solution_size(&config);
 // result.is_valid: bool