⚡ Bolt: Optimize JSON stringify allocations

[Tcode-Motion]

💡 What:
Optimized stringify_value in techscript_stdlib JSON module to write directly to a single mutable string buffer.

🎯 Why:
The previous implementation used format! and joining Vec<String> allocations recursively for JSON lists and maps. This caused significant unnecessary memory allocations and CPU overhead in the hot path of JSON serialization.

📊 Impact:
Reduces execution time for stringify_value by approximately ~76% and entirely removes intermediate allocations for array and object children strings.

🔬 Measurement:
Verified through cargo bench -p techscript_stdlib json indicating a reduction from ~765 ns to ~182 ns for a sample map stringification. Verified through cargo test -p techscript_stdlib json ensuring 100% test compatibility.

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PR created automatically by Jules for task 1696979005875390941 started by @Tcode-Motion

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