chore: code audit improvements and ci infrastructure - #24

Summary

Comprehensive code audit addressing infrastructure gaps and code quality improvements.

Infrastructure additions

• rust-toolchain.toml: pins Rust 1.93 with fmt/clippy components for reproducible builds
• .cargo/config.toml: release optimizations (LTO, strip symbols, single codegen unit)
• .github/workflows/ci.yml: automated CI with format checks, clippy, tests (ubuntu + macos), and security audits via cargo-audit
• CONTRIBUTING.md: development workflow and code standards for contributors

Code improvements

• glob_match(): rewrote with O(n*m) iterative two-pointer algorithm instead of O(n²) recursive backtracking — prevents potential DOS on patterns like *a*b*c*d
• SCAN cursor validation: added guard against invalid cursor values (shard_id out of range)
• documentation: improved docs for memory overhead constants, LRU sampling approximation, and SCAN cursor encoding format

Test plan

• all 410 tests pass
• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• glob pattern tests verify new algorithm handles all cases

Design considerations

The glob_match rewrite uses an iterative backtracking approach that:

1. Records the position of each * encountered
2. On mismatch, backtracks to the last * and tries consuming one more char
3. Linear in the number of stars, avoiding exponential blowup

This is the same algorithm used by Redis and most shell glob implementations.