perf: optimize shell startup with compinit caching, zwc bytecode, and zsh-defer

[sha1n]

Summary

• Cache compinit dump file and skip regeneration when fresh (<24h)
• Compile zsh source files to .zwc bytecode for faster sourcing
• Defer non-critical plugins (zsh-autosuggestions, zsh-syntax-highlighting, zsh-history-substring-search) via zsh-defer
• Add zsh_bench script for measuring shell startup time
• Add make compile / make clean_zwc / reload! for bytecode management
• Add .benchmarks/ to global gitignore

Test plan

• Run make test to verify nothing is broken
• Run zsh_bench to confirm startup benchmarking works
• Open a new shell and verify deferred plugins load correctly (autosuggestions, syntax highlighting, history search)
• Run make compile and make clean_zwc to verify bytecode lifecycle

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