docs: add docs/CODING-STYLE.md Rust style guide

Author: lvyaoyuan168-alt

docs/CODING-STYLE.md

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+# Coding Style
+
+## Formatting
+- cargo fmt is canonical
+- 4-space indent
+- Max line 100 chars
+
+## Patterns
+- Prefer ? over match for error propagation
+- Use anyhow for app errors, thiserror for library errors
+- Avoid unwrap() in production paths; use expect() with context
+
+## Naming
+- snake_case for fns/vars, PascalCase for types
+- Single-letter lifetimes only in small scopes