⚡ Bolt: Use write! macro to prevent temporary allocations in canvas

[Tcode-Motion]

💡 What: Replaced all instances of push_str(&format!(...)) with let _ = write!(...); in stdlib/src/canvas.rs to write directly to the target String buffer. Added comments explaining the optimization. Appended learning to .jules/bolt.md.
🎯 Why: push_str(&format!(...)) creates a temporary String allocation for every concatenation, which is especially wasteful inside loops (like rendering multiple rings or paths). Writing directly to the buffer avoids these intermediate allocations.
📊 Impact: Reduces memory allocations during SVG canvas generation and DSL block rendering.
🔬 Measurement: Code compiles successfully, and all standard library tests (cargo test --workspace) pass without regressions, ensuring identical SVG output string generation but with fewer internal allocations.

---

PR created automatically by Jules for task 5041535596994256397 started by @Tcode-Motion

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.