test: make zig build test actually run all tests + fix latent rot

root.zig had no `test` block, so the test binary discovered zero tests and
trivially "passed" — every src test had silently rotted. Add
`refAllDecls(@This())` to root.zig so all 185 tests run, then fix the rot it
surfaced:

- emit_llvm.test: operands were constants, so LLVM folded the very
  instructions being asserted (fadd/sub/icmp/insertvalue/extractvalue/sext).
  Rewrite to use function-parameter operands; `main` now returns i32 (entry
  convention); tagged-union enum_init lowers via memory, not insertvalue.
- interp.test: switch the per-test allocator to an arena (the interpreter is
  arena-style and intentionally frees little) — clears the transient-Value
  leaks without an ownership-ambiguous source change.
- lower.test: pass `is_imported` to lowerFunction; mark two helpers `pub`; the
  if/else block test now uses a runtime (param) condition since lowering folds
  `if true`.
- print.test: SSA numbering — params occupy %0/%1, so consts start at %2.
- jni_java_emit.test: nested-class refs render in Java source form
  (`SurfaceHolder.Callback`), not the JNI `$` form.

Leaks fixed at the source where ownership was clear: Module gains an arena for
the operand slices the Builder dupes (struct/call/branch/switch args, block
params, lowerFunction params); objcDefinedStateStructType builds its field
slice in that arena and frees its temp name string.

src/ir/jni_java_emit.test.zig

@@ -342,9 +342,11 @@ test "#implements clauses on the class header" {
     const out = try emit.emitJavaSource(a, &fcd, .{ .classes = &registry });
     defer a.free(out);
 
+    // Registry value `android/view/SurfaceHolder$Callback` is emitted in Java
+    // *source* form: `/` → `.` and the nested-class `$` → `.`.
     try std.testing.expect(std.mem.indexOf(
         u8,
         out,
-        "public class SxApp extends android.app.Activity implements android.view.SurfaceHolder$Callback, java.lang.Runnable {",
+        "public class SxApp extends android.app.Activity implements android.view.SurfaceHolder.Callback, java.lang.Runnable {",
     ) != null);
 }