Unified diagnostic codes for the dmc pipeline.
Every layer (lexer, parser, transform, codegen) emits into one shared
DiagnosticEngine<Code>. Per-layer variants are gated behind cargo
features so a crate that only needs lexer codes can opt out of the rest.
Feature flags
lexer-E***lexer-emitted variantsparser-P***/PW***parser-emitted variantstransform-T***/TW***transform-emitted variantscodegen-G***/GW***codegen-emitted variants
A normal full build (e.g. via dmc-core) enables all features.