⚡ Bolt: Optimize recursive string allocations in canvas SVG rendering

[Tcode-Motion]

💡 What: Replaced recursive String returns in Canvas DSL rendering with a mutable &mut String buffer parameter.
🎯 Why: Deeply nested structures generated a new String at each level, causing severe memory allocation overhead during concatenation. Reusing a single buffer avoids these intermediate allocations.
📊 Measured Improvement: Reduces String allocations by O(N) where N is the number of DSL blocks processed, substantially improving speed and lowering memory usage.

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PR created automatically by Jules for task 6665494219215426163 started by @Tcode-Motion

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