⚡ Bolt: [performance improvement] Hoist CSS and SHA-256 generation out of process_dir

[seonghobae]

💡 What: Hoisted static CSP style strings (cssContent, css) and expensive cryptographic SHA-256 hash generation out of the recursive process_dir function, declaring them as top-level private val properties.
🎯 Why: process_dir is called for every single directory traversed. Re-allocating the large CSS string and re-computing the SHA-256 hash dynamically on every recursive call created severe, unnecessary CPU and memory overhead during tree generation.
📊 Impact: Expected to reduce tree generation CPU time and memory allocation significantly on directories with many subdirectories, as string allocation and cryptographic hashing now only happen once at class load time rather than N times.
🔬 Measurement: Verify tests run properly and memory usage/GC pressure is reduced when running html4tree on large directories (e.g. root file systems).

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PR created automatically by Jules for task 2433005278948987050 started by @seonghobae

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