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/lexer.rs

@@ -588,8 +588,8 @@ mod tests {
         assert_eq!(
             kinds(src),
             vec![
-                PlusEq, MinusEq, StarEq, SlashEq, PercentEq, AmpEq, PipeEq, CaretEq, ShlEq,
-                ShrEq, Eof
+                PlusEq, MinusEq, StarEq, SlashEq, PercentEq, AmpEq, PipeEq, CaretEq, ShlEq, ShrEq,
+                Eof
             ]
         );
     }