Add expression AST and Pratt parser with REPL

- ast.rs: Expr/ExprKind with UnaryOp, BinaryOp, StructField
- parser.rs: Pratt expression parser with allow_struct_literals flag,
  error recovery via dummy tokens, and 19 unit tests
- main.rs: interactive expression REPL (prints parsed AST)

fluxc/src/ast.rs

@@ -0,0 +1,113 @@
+use crate::token::Span;
+
+// ── Operators ──────────────────────────────────────────────────────────────────
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum UnaryOp {
+    Neg,    // `-`
+    Not,    // `!`
+    BitNot, // `~`
+    Deref,  // `*`
+    AddrOf, // `&`
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BinaryOp {
+    // Logical
+    Or,  // `or`
+    And, // `and`
+    // Bitwise
+    BitOr,  // `|`
+    BitXor, // `^`
+    BitAnd, // `&`
+    // Comparison
+    Eq, // `==`
+    Ne, // `!=`
+    Lt, // `<`
+    Gt, // `>`
+    Le, // `<=`
+    Ge, // `>=`
+    // Arithmetic
+    Add, // `+`
+    Sub, // `-`
+    Mul, // `*`
+    Div, // `/`
+    Rem, // `%`
+}
+
+// ── Struct literal field ───────────────────────────────────────────────────────
+
+#[derive(Debug, Clone)]
+pub struct StructField {
+    pub name: String,
+    pub name_span: Span,
+    pub value: Expr,
+}
+
+// ── Expression ────────────────────────────────────────────────────────────────
+
+#[derive(Debug, Clone)]
+pub struct Expr {
+    pub kind: ExprKind,
+    pub span: Span,
+}
+
+impl Expr {
+    pub fn new(kind: ExprKind, span: Span) -> Self {
+        Self { kind, span }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum ExprKind {
+    // Literals
+    IntLit(String),
+    FloatLit(String),
+    StringLit(String),
+    CharLit(String),
+    Bool(bool),
+
+    // Identifier
+    Ident(String),
+
+    // Struct literal: `Foo { x: 1, y: 2 }`
+    StructLit {
+        name: String,
+        name_span: Span,
+        fields: Vec<StructField>,
+    },
+
+    // Operators
+    Unary {
+        op: UnaryOp,
+        op_span: Span,
+        expr: Box<Expr>,
+    },
+    Binary {
+        op: BinaryOp,
+        op_span: Span,
+        lhs: Box<Expr>,
+        rhs: Box<Expr>,
+    },
+
+    // Postfix
+    Field {
+        expr: Box<Expr>,
+        field: String,
+        field_span: Span,
+    },
+    Index {
+        expr: Box<Expr>,
+        index: Box<Expr>,
+    },
+    Call {
+        callee: Box<Expr>,
+        args: Vec<Expr>,
+    },
+
+    // Parenthesised expression
+    Group(Box<Expr>),
+
+    // Placeholder for parse errors — allows parsing to continue
+    Error,
+}