check: mode-strict-because suppression (iter 19b)

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).

crates/ailang-core/src/ast.rs

@@ -183,6 +183,41 @@ pub struct FnDef {
     /// Optional source-level documentation string.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub doc: Option<String>,
+    /// Iter 19b: structured-diagnostic suppressions opted into for this
+    /// fn. Each entry names a diagnostic code and an author-asserted
+    /// reason. Currently the only consumer is `over-strict-mode`
+    /// (Iter 19a / 19a.1) but the mechanism is generic across codes.
+    /// `because` must be non-empty — the typechecker emits
+    /// `empty-suppress-reason` (Error) otherwise.
+    ///
+    /// Serialised with `skip_serializing_if = "Vec::is_empty"` so
+    /// every pre-19b fixture's canonical-JSON hash stays bit-identical.
+    /// The same additive-schema pattern is used by [`TypeDef::vars`]
+    /// (Iter 13a) and [`Type::Con::args`] (Iter 13a).
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub suppress: Vec<Suppress>,
+}
+
+/// Iter 19b: one entry in [`FnDef::suppress`]. Marks a structured
+/// diagnostic the author has consciously decided to allow on this
+/// def, with a mandatory reason.
+///
+/// `because` is non-empty by schema rule — the typechecker emits
+/// `empty-suppress-reason` (Error severity) when it is empty or
+/// whitespace-only, and a wrong/unknown `code` simply matches no
+/// diagnostic and therefore suppresses nothing (the original
+/// diagnostic still fires unmasked).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Suppress {
+    /// The diagnostic code being suppressed (e.g.
+    /// `"over-strict-mode"`). Matched against
+    /// [`crate::SCHEMA`]-side codes; an unknown code suppresses
+    /// nothing but is not itself an error (the diagnostic registry
+    /// is open-set).
+    pub code: String,
+    /// The author's stated reason. Must be non-empty — the
+    /// typechecker emits `empty-suppress-reason` (Error) otherwise.
+    pub because: String,
 }
 
 /// A constant (top-level binding to a value).