Iter 16e: == polymorphic over Int / Bool / Str / Unit

Lifts the Int-only restriction on `==`. Declared type becomes
forall a. Fn(a, a) -> Bool; codegen monomorphises and dispatches:

  Int  → icmp eq i64
  Bool → icmp eq i1
  Str  → call @strcmp + icmp eq i32 0
  Unit → constant true (operands still emitted for side effects)
  ADT / Fn / other → CodegenError::Internal

This unblocks 16c's build_eq for non-Int lit patterns. == joins
__unreachable__ as the second polymorphic builtin (same Forall
machinery).

- check/builtins.rs: == registered as Forall(a, Fn(a, a) -> Bool).
- codegen: lower_eq dispatch table; @strcmp declared in IR header
  alongside @printf/@GC_malloc/@puts.
- examples/eq_demo.{ailx,ail.json}: covers all four supported
  scalars including a Str-lit-pattern match.
- IR snapshots refreshed: only +declare i32 @strcmp(ptr, ptr) in
  the header; every define body bit-identical.
- e2e + check + codegen tests: 124 → 133 (+9, of which +1 is the
  e2e fixture and the rest exercise the new dispatch / typecheck
  surface).

Other comparison ops (<, <=, >, >=, !=) remain Int-only — out of
scope for this iter.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ail/tests/e2e.rs

@@ -1137,3 +1137,29 @@ fn unreachable_demo() {
     let lines: Vec<&str> = stdout.lines().collect();
     assert_eq!(lines, vec!["4", "5"]);
 }
+
+/// Iter 16e: polymorphic `==`. Properties protected:
+/// (1) `==` typechecks at Int / Bool / Str / Unit (the fixture
+///     uses every supported case directly via `(app == ...)`);
+/// (2) codegen dispatches each arg-type to the right LLVM shape
+///     — `icmp eq i64`, `icmp eq i1`, `@strcmp + icmp eq i32 0`,
+///     constant `i1 true` — and the binary's stdout matches the
+///     boolean truth-table for those operators;
+/// (3) 16c's `build_eq` desugar of `(pat-lit "hi")` over a `Str`
+///     scrutinee now goes through, since `==` no longer rejects
+///     non-Int args at typecheck.
+#[test]
+fn eq_demo() {
+    let stdout = build_and_run("eq_demo.ail.json");
+    let lines: Vec<&str> = stdout.lines().collect();
+    assert_eq!(
+        lines,
+        vec![
+            "true", "false", // == on Int
+            "false", "true", // == on Bool
+            "true", "false", // == on Str
+            "true", // == on Unit
+            "1", "2", "0", // classify_str: Str lit-pattern desugar
+        ]
+    );
+}

crates/ail/tests/snapshots/hello.ll

@@ -7,6 +7,7 @@ target triple = "<NORMALIZED>"
 declare i32 @printf(ptr, ...)
 declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
+declare i32 @strcmp(ptr, ptr)
 
 @ail_hello_main_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_hello_main_adapter, ptr null }
 define i8 @ail_hello_main() {

crates/ail/tests/snapshots/list.ll

@@ -7,6 +7,7 @@ target triple = "<NORMALIZED>"
 declare i32 @printf(ptr, ...)
 declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
+declare i32 @strcmp(ptr, ptr)
 
 @ail_list_sum_list_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_list_sum_list_adapter, ptr null }
 @ail_list_main_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_list_main_adapter, ptr null }

crates/ail/tests/snapshots/max3.ll

@@ -7,6 +7,7 @@ target triple = "<NORMALIZED>"
 declare i32 @printf(ptr, ...)
 declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
+declare i32 @strcmp(ptr, ptr)
 
 @ail_max3_max_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_max3_max_adapter, ptr null }
 @ail_max3_max3_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_max3_max3_adapter, ptr null }

crates/ail/tests/snapshots/sum.ll

@@ -7,6 +7,7 @@ target triple = "<NORMALIZED>"
 declare i32 @printf(ptr, ...)
 declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
+declare i32 @strcmp(ptr, ptr)
 
 @ail_sum_sum_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_sum_sum_adapter, ptr null }
 @ail_sum_main_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_sum_main_adapter, ptr null }

crates/ail/tests/snapshots/ws_main.ll

@@ -7,6 +7,7 @@ target triple = "<NORMALIZED>"
 declare i32 @printf(ptr, ...)
 declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
+declare i32 @strcmp(ptr, ptr)
 
 @ail_ws_lib_add_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_ws_lib_add_adapter, ptr null }
 @ail_ws_main_main_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_ws_main_main_adapter, ptr null }