test: red for Pattern::Lit::Float not rejected through full check pipeline

Pins fieldtest finding B1 (docs/specs/2026-05-10-fieldtest-floats.md).
DESIGN.md and JOURNAL Floats.3 promise that pattern-matching on a
Float literal is hard-rejected at typecheck via
CheckError::FloatPatternNotAllowed, but the rejection only fires when
typecheck is reached directly. The full check pipeline runs
ailang_core::desugar::desugar_module first, and build_eq rewrites
Pattern::Lit { Literal::Float } arms into (== scrutinee 1.5_FLOAT)
before typecheck — so the existing iter-3.4 reject arm at lib.rs:2316
is unreachable on input flowing through check / check_module /
check_workspace.

The existing reject_float_pattern_in_match test in builtins.rs calls
synth(...) directly on a hand-built Term::Match, bypassing desugar; it
passes today but does not protect the spec'd property. This new test
exercises check(&m) end-to-end and asserts the FloatPatternNotAllowed
error — currently fails because check returns Ok(CheckedModule).

GREEN side handed off to skills/implement mini-mode.

crates/ailang-check/src/lib.rs

@@ -4468,4 +4468,67 @@ mod tests {
         ailang_surface::parse_term(rep)
             .unwrap_or_else(|e| panic!("suggested rewrite must parse: {rep} ({e})"));
     }
+
+    /// Floats milestone post-fieldtest B1 (RED): pattern-matching on a
+    /// `Literal::Float` literal must be hard-rejected at typecheck via
+    /// `CheckError::FloatPatternNotAllowed` per DESIGN.md §"Float
+    /// semantics" — even when the match arm reaches `check` through
+    /// the *full* check pipeline (which runs `desugar_module` first).
+    ///
+    /// The existing `reject_float_pattern_in_match` test in
+    /// `crates/ailang-check/src/builtins.rs` calls `synth(...)`
+    /// directly on a hand-built `Term::Match`, bypassing desugar; it
+    /// passes today but does not protect the property the spec
+    /// promises. The desugar pass `build_eq` rewrites
+    /// `Pattern::Lit { lit }` arms (including `Literal::Float`) into
+    /// `Term::If { cond: (== scrutinee lit), .. }` *before* typecheck
+    /// runs (see `crates/ailang-core/src/desugar.rs:1056`), so the
+    /// `Pattern::Lit { lit: Literal::Float { .. } }` arm at
+    /// `lib.rs:2316` is unreachable on input that flows through
+    /// `check`/`check_module`/`check_workspace`. This test exercises
+    /// the full pipeline and pins the Float-pattern hard-reject as
+    /// a property of the public check API.
+    #[test]
+    fn check_rejects_float_pattern_through_full_pipeline() {
+        // fn classify : (Float) -> Int = \x. match x { 0.0 -> 1; _ -> 0 }
+        let bits = 0.0_f64.to_bits();
+        let m = Module {
+            schema: SCHEMA.into(),
+            name: "t".into(),
+            imports: vec![],
+            defs: vec![fn_def(
+                "classify",
+                Type::Fn {
+                    params: vec![Type::float()],
+                    ret: Box::new(Type::int()),
+                    effects: vec![],
+                    param_modes: vec![],
+                    ret_mode: ParamMode::Implicit,
+                },
+                vec!["x"],
+                Term::Match {
+                    scrutinee: Box::new(Term::Var { name: "x".into() }),
+                    arms: vec![
+                        Arm {
+                            pat: Pattern::Lit {
+                                lit: Literal::Float { bits },
+                            },
+                            body: Term::Lit { lit: Literal::Int { value: 1 } },
+                        },
+                        Arm {
+                            pat: Pattern::Wild,
+                            body: Term::Lit { lit: Literal::Int { value: 0 } },
+                        },
+                    ],
+                },
+            )],
+        };
+        let err = check(&m).expect_err(
+            "Pattern::Lit { Literal::Float } must be rejected at typecheck",
+        );
+        assert!(
+            matches!(err, CheckError::FloatPatternNotAllowed),
+            "expected CheckError::FloatPatternNotAllowed, got {err:?}"
+        );
+    }
 }