docs(rust-style): order declarations by dependency

src/skills/style/rust/rules/declarations.md

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+# Declaration rules
+
+## Order declarations by dependency
+
+### Rule
+
+Order Rust declarations in depth-first dependency order.
+Place each function, struct, and enum before private declarations that support it.
+For structs and enums, order supporting declarations by field or variant order.
+For functions, order helper declarations by first use in the function body.
+Apply the same rule recursively to each supporting declaration.
+
+### Rationale
+
+Dependency order lets readers start at the public operation or type.
+Readers can then continue downward into the declarations that explain each part.
+
+### Examples
+
+```rust
+pub struct Report {
+    files: FileStore,
+    diagnostics: Vec<Diagnostic>,
+}
+
+struct FileStore {
+    files: Vec<SourceFile>,
+}
+
+struct SourceFile {
+    path: PathBuf,
+    source: String,
+}
+
+struct Diagnostic {
+    message: String,
+}
+```