Iter 16c — literal patterns via desugar

Desugar `Pattern::Lit` (top-level and nested in Ctor) to
`Term::If { cond = (== sv lit), then = body, else_ = fall_k }`.
After 16c, no Pattern::Lit survives the desugar pass; codegen
and typechecker never see one. `is_flat` reclassifies Lit as
non-flat to force the chain machinery; new `build_eq` helper
constructs the equality test per literal kind.

Tests: 99 → 103 (+1 e2e lit_pat_demo, +3 desugar unit). Demo
fixture exercises top-level lit arms (classify) and nested
lit-in-Ctor (categorize_first) with a local IntList; output
100/200/999/-1/0/7.

Known limitations documented in DESIGN.md and JOURNAL: (a)
chain machinery's Unit terminator means non-Wild-terminated
exhaustive matches with lit arms still need a trailing `_`
catch-all to type-check; (b) `==` is currently Int-only, so
Bool/Str/Unit lit patterns desugar correctly but error at
typecheck.

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

crates/ail/tests/e2e.rs

@@ -960,3 +960,18 @@ fn check_json_unbound_var() {
         "expected at least one error diagnostic with code unbound-var; got: {stdout}"
     );
 }
+
+/// Iter 16c: literal patterns at top level and inside Ctor
+/// sub-patterns. Property protected: the 16a desugar pass rewrites
+/// every `Pattern::Lit` to a `Term::If` against the scrutinee/field
+/// before either typecheck or codegen sees it. Without 16c, the
+/// codegen would emit a Match arm with a Lit pattern and produce
+/// either a wrong result or an `unreachable!` panic depending on
+/// the path. The fixture exercises both sites: `classify` (top-
+/// level lit arms) and `categorize_first` (nested lit-in-Ctor).
+#[test]
+fn lit_pat_demo() {
+    let stdout = build_and_run("lit_pat.ail.json");
+    let lines: Vec<&str> = stdout.lines().collect();
+    assert_eq!(lines, vec!["100", "200", "999", "-1", "0", "7"]);
+}