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.lock

@@ -7,6 +7,7 @@ name = "fluxc"
 version = "0.1.0"
 dependencies = [
  "unicode-xid",
+ "yansi",
 ]
 
 [[package]]
@@ -14,3 +15,9 @@ name = "unicode-xid"
 version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"
+
+[[package]]
+name = "yansi"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cfe53a6657fd280eaa890a3bc59152892ffa3e30101319d168b781ed6529b049"