Iter 7: first-class function references (no capture)

Top-level fn names are now usable as values, fn-typed parameters can
be called as `f(args)`. KISS slice of "closures + HOFs + TIR": no
TIR, no heap, no ABI shift — that bundle stays in Iter 8 where
closure-with-capture and lambdas land together.

Codegen:
- Type::Fn lowers to LLVM `ptr`; sig travels via a per-fn-body
  `ssa_fn_sigs` sidetable on `Emitter`.
- Term::Var falls through to `resolve_top_level_fn` when the name is
  not a local; emits `@ail_<m>_<def>` and registers the sig.
- Term::App splits: static callee (builtin / current-module / qualified)
  keeps the existing direct path; otherwise lower the callee, look up
  the sig, emit indirect `call <ret> (<param-tys>) %fn(args...)`.
- emit_fn registers fn-typed params in the sidetable on entry.
- If branches propagate a fn-pointer sig to their phi when both arms
  match.

Typechecker: unchanged. It already accepted `synth(callee)` against
Type::Fn; the only blocker was codegen rejecting non-Var callees.

Tests: 48 green (was 47). New `examples/hof.ail.json` and e2e
`higher_order_apply_inc` exercise `apply(inc, 41) == 42` end-to-end.

DESIGN.md: "What is not (yet) supported" rewritten — closures-with-
capture and lambdas remain pending, first-class fn-refs added as
positive bullet. Smoke-test list extended with `hof.ail.json`.

JOURNAL.md: Iter 7 entry with rationale, scope choice, architecture
self-check, Iter 8 plan.

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

crates/ail/tests/e2e.rs

@@ -64,6 +64,14 @@ fn list_sum_via_match() {
     assert_eq!(stdout.trim(), "42");
 }
 
+/// Iter 7: first-class function references — `apply(inc, 41)` must
+/// produce 42, exercising fn-typed parameters and indirect call.
+#[test]
+fn higher_order_apply_inc() {
+    let stdout = build_and_run("hof.ail.json");
+    assert_eq!(stdout.trim(), "42");
+}
+
 /// Guards `ail diff`: a modified body changes the hash of `sum`, while
 /// `main` stays unchanged. Expects exit code 1, `changed` contains exactly
 /// `sum`, `unchanged` contains `main`, `added`/`removed` empty.