Iter 16d: chain-terminator via __unreachable__ builtin

Eliminates the Unit-typed chain default that 16c left in place.
Adds __unreachable__ : forall a. a — codegen lowers to LLVM
unreachable. The 16a/16c chain machinery now uses it as the
deepest fall-through, so exhaustive matches with non-Unit arms
no longer need a (case _ ...) workaround.

- check/builtins.rs: register __unreachable__ as Forall(a, a).
- codegen: Term::Var "__unreachable__" emits LLVM unreachable
  and sets block_terminated; If/Match/fn-body gate downstream
  work on that flag.
- desugar: chain default switched from Lit{Unit} to Var.
- examples/lit_pat: categorize_first's trailing _ arm removed.
  Hash changes from c4faec3abc2ed388 to 644de0c0ec15fc17.
- examples/unreachable_demo: positive fixture using safe_div
  with __unreachable__ in the impossible branch.
- e2e + desugar tests: 122 → 124 (+2).

Path-a from 16b.2 planning: real primitive over a desugar-time
exhaustiveness pre-check (which would need cross-module type
registry access from desugar).

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

crates/ailang-check/src/builtins.rs

@@ -73,6 +73,20 @@ pub fn install(env: &mut crate::Env) {
         },
     );
 
+    // Iter 16d: `__unreachable__` is the polymorphic bottom value.
+    // Type: `forall a. a`. Used by the desugar pass as the chain
+    // terminator of an exhaustive match, and available to user code as
+    // a primitive panic point. Codegen lowers it to LLVM `unreachable`.
+    // It is a value, not a fn — reference site is `(var __unreachable__)`,
+    // not `(app __unreachable__)`.
+    env.globals.insert(
+        "__unreachable__".into(),
+        Type::Forall {
+            vars: vec!["a".into()],
+            body: Box::new(Type::Var { name: "a".into() }),
+        },
+    );
+
     env.effect_ops.insert(
         "io/print_int".into(),
         EffectOpSig {
@@ -130,6 +144,7 @@ pub fn list() -> Vec<(&'static str, &'static str)> {
         (">", "(Int, Int) -> Bool"),
         (">=", "(Int, Int) -> Bool"),
         ("not", "(Bool) -> Bool"),
+        ("__unreachable__", "forall a. a"),
         ("io/print_int", "(Int) -> Unit !IO  [effect op]"),
         ("io/print_bool", "(Bool) -> Unit !IO  [effect op]"),
         ("io/print_str", "(Str) -> Unit !IO  [effect op]"),