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.

examples/invalid.flx

@@ -0,0 +1,4 @@
+fn main() {
+    let a : 12 = 3;
+    return -;
+}