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/Cargo.toml

@@ -5,3 +5,4 @@ edition = "2024"
 
 [dependencies]
 unicode-xid = "0.2"
+yansi = "1"