Add Rust language documentation

[mahendrashelke]

Summary

• Adds LLM-optimized Rust language guide at content/rust-lang/docs/rust/rust/DOC.md
• Covers common AI agent pitfalls: pattern matching, numeric casting, checked arithmetic, ownership/borrowing, error handling, enums for domain logic, concurrency, and edition 2024 changes
• Incorporates security patterns from Pitfalls of Safe Rust: TOCTOU, constant-time comparison, unbounded input, cargo-geiger
• Incorporates bad habits from Bad Habits from Other Languages
• 394 lines, validates clean with chub build --validate-only
• Pure Rust std library — no external crate dependencies in examples

Test plan

• node cli/bin/chub build content/ --validate-only passes (1554 docs, 0 warnings)
• Frontmatter follows CONTRIBUTING.md format
• Under 500 line limit

[mahendrashelke]

cc @rohitprasad15 — would appreciate a review when you get a chance!

[Copilot]

Pull request overview

Adds a new Rust language guideline document intended to reduce common incorrect patterns produced by AI agents, with emphasis on ownership/borrowing, error handling, safety/security pitfalls, concurrency, and edition 2024 notes.

Changes:

• Introduces a Rust language guidelines doc with “golden rules” and common pitfalls.
• Adds security-focused guidance (TOCTOU, constant-time comparison, unbounded input) and unsafe-code hygiene tips.
• Includes examples covering error handling, domain modeling with enums, and basic stdlib concurrency.

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