50b68267fe
Closes the 19a/19a.1/19b arc. Corpus signal from 19a.1 (5/65
fixtures fire over-strict-mode, all deliberate RC codegen-test
fixtures) justified shipping the suppress mechanism end-to-end.
Schema: Suppress { code, because } on FnDef. Pre-19b hashes
bit-identical via skip_serializing_if. Typechecker drops matching
diagnostics; empty 'because' is Error severity; wrong codes are
silent no-ops (open-set registry).
Form-A: (suppress (code "...") (because "...")) clause, parser
+ printer round-trip clean. Form-B: '// @suppress <code>: <reason>'
above the doc string, lossless contract metadata.
5 RC fixtures migrated (.ail.json + .ailx + .prose.txt). Corpus
signal: 5/65 -> 0/65. Lint still fires on any future fn that's
accidentally over-strict without an authored reason.
Test counts: ailang-check 55->61, ailang-core 26->28, ailang-surface
21->26, ailang-prose 49->52, e2e 70 unchanged.
Known debt: .ailx comment headers lost on regen (ail render's
contract excludes comments); parse_suppress_attr accepts
duplicate code/because attrs without diagnose (bounded by canonical
print order).