Add function/struct definition parsing and program entry point

- ast.rs: Param, FieldDef, FuncDef, StructDef, TopLevelDef,
  TopLevelDefKind, Program
- parser.rs: parse_program, parse_top_level_def, parse_func_def,
  parse_struct_def with param/field list helpers;
  synchronize_top_level for recovery; 14 new tests (76 total)
- main.rs: parse source file as a Program and print the AST

fluxc/src/ast.rs

@@ -201,3 +201,64 @@ pub enum StmtKind {
     /// Error placeholder — emitted during recovery so the parent can continue.
     Error,
 }
+
+// ── Top-level definitions ──────────────────────────────────────────────────────
+
+/// A function parameter: `[mut] name : type`.
+#[derive(Debug, Clone)]
+pub struct Param {
+    pub mutable: bool,
+    pub name: String,
+    pub name_span: Span,
+    pub ty: Type,
+}
+
+/// A struct definition field: `name : type`.
+///
+/// Named `FieldDef` to distinguish from `StructField`, which is a
+/// field in a struct *literal expression*.
+#[derive(Debug, Clone)]
+pub struct FieldDef {
+    pub name: String,
+    pub name_span: Span,
+    pub ty: Type,
+}
+
+/// `fn name ( params ) [ -> type ] block`
+#[derive(Debug, Clone)]
+pub struct FuncDef {
+    pub name: String,
+    pub name_span: Span,
+    pub params: Vec<Param>,
+    pub ret_ty: Option<Type>,
+    pub body: Block,
+}
+
+/// `struct name { fields }`
+#[derive(Debug, Clone)]
+pub struct StructDef {
+    pub name: String,
+    pub name_span: Span,
+    pub fields: Vec<FieldDef>,
+}
+
+#[derive(Debug, Clone)]
+pub struct TopLevelDef {
+    pub kind: TopLevelDefKind,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub enum TopLevelDefKind {
+    Func(FuncDef),
+    Struct(StructDef),
+    /// Error placeholder for recovery.
+    Error,
+}
+
+/// The root of the AST — a sequence of top-level definitions.
+#[derive(Debug, Clone)]
+pub struct Program {
+    pub defs: Vec<TopLevelDef>,
+    pub span: Span,
+}