Feat: add structured diagnostics with yansi colors

Introduces fluxc/src/diagnostics.rs with Level (Critical, Error,
Warning, Note), Label (Primary/Secondary with optional message), and
Diagnostic types. Diagnostic::render(src, filename) produces
rustc-style output: colored header, --> file:line:col pointer, source
line with gutter, and ^ / - underlines aligned to the offending span.

Replaces the flat ParseError struct in the parser; all five error
sites now emit Diagnostic values with source-pointing labels.

fluxc/src/parser.rs

@@ -1,29 +1,14 @@
-use std::fmt;
-
 use crate::{
     ast::{
         BinaryOp, Block, CompoundAssignOp, ElseBranch, Expr, ExprKind, FieldDef, FuncDef, Param,
         Program, Stmt, StmtKind, StructDef, StructField, TopLevelDef, TopLevelDefKind, Type,
         UnaryOp,
     },
+    diagnostics::{Diagnostic, Label},
     lexer::Lexer,
     token::{Span, Token, TokenKind},
 };
 
-// ── Parse error ───────────────────────────────────────────────────────────────
-
-#[derive(Debug, Clone)]
-pub struct ParseError {
-    pub span: Span,
-    pub message: String,
-}
-
-impl fmt::Display for ParseError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "error at {}: {}", self.span, self.message)
-    }
-}
-
 // ── Binding powers ─────────────────────────────────────────────────────────────
 //
 // Returns `(left_bp, right_bp)` for infix operators.
@@ -139,7 +124,7 @@ fn token_to_binary_op(kind: TokenKind) -> BinaryOp {
 pub struct Parser<'src> {
     tokens: Vec<Token<'src>>,
     pos: usize,
-    pub errors: Vec<ParseError>,
+    pub errors: Vec<Diagnostic>,
 }
 
 impl<'src> Parser<'src> {
@@ -175,11 +160,12 @@ impl<'src> Parser<'src> {
         if tok.kind == kind {
             self.advance()
         } else {
+            self.errors.push(
+                Diagnostic::error(format!("expected {kind}, found {}", tok.kind)).with_label(
+                    Label::primary(tok.span).with_message(format!("expected {kind} here")),
+                ),
+            );
             let span = Span::new(tok.span.start, tok.span.start);
-            self.errors.push(ParseError {
-                span,
-                message: format!("expected {}, found {}", kind, tok.kind),
-            });
             Token::new(kind, span, "")
         }
     }
@@ -261,10 +247,10 @@ impl<'src> Parser<'src> {
 
             // Error — insert recovery placeholder
             _ => {
-                self.errors.push(ParseError {
-                    span: tok.span,
-                    message: format!("expected type, found {}", tok.kind),
-                });
+                self.errors.push(
+                    Diagnostic::error(format!("expected type, found {}", tok.kind))
+                        .with_label(Label::primary(tok.span).with_message("expected a type here")),
+                );
                 Type::Error
             }
         }
@@ -339,10 +325,10 @@ impl<'src> Parser<'src> {
             | TokenKind::Fn
             | TokenKind::Struct
             | TokenKind::Eof => {
-                self.errors.push(ParseError {
-                    span: tok.span,
-                    message: format!("unexpected {} in statement position", tok.kind),
-                });
+                self.errors.push(
+                    Diagnostic::error(format!("unexpected {} in statement position", tok.kind))
+                        .with_label(Label::primary(tok.span)),
+                );
                 self.synchronize();
                 Stmt {
                     kind: StmtKind::Error,
@@ -553,10 +539,10 @@ impl<'src> Parser<'src> {
 
             // Error recovery
             _ => {
-                self.errors.push(ParseError {
-                    span: tok.span,
-                    message: format!("unexpected token {} in expression", tok.kind),
-                });
+                self.errors.push(
+                    Diagnostic::error(format!("unexpected token {} in expression", tok.kind))
+                        .with_label(Label::primary(tok.span)),
+                );
                 Expr::new(ExprKind::Error, tok.span)
             }
         }
@@ -730,10 +716,12 @@ impl<'src> Parser<'src> {
             TokenKind::Fn => self.parse_func_def(),
             TokenKind::Struct => self.parse_struct_def(),
             _ => {
-                self.errors.push(ParseError {
-                    span: tok.span,
-                    message: format!("expected `fn` or `struct`, found {}", tok.kind),
-                });
+                self.errors.push(
+                    Diagnostic::error(format!("expected `fn` or `struct`, found {}", tok.kind))
+                        .with_label(
+                            Label::primary(tok.span).with_message("expected `fn` or `struct`"),
+                        ),
+                );
                 self.synchronize_top_level();
                 TopLevelDef {
                     kind: TopLevelDefKind::Error,