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/root.zig

@@ -20,3 +20,12 @@ pub const lsp = struct {
     pub const types = @import("lsp/types.zig");
     pub const document = @import("lsp/document.zig");
 };
+
+test {
+    // Discover every test in the module graph so `zig build test` actually
+    // runs them. Without this, the test binary finds no `test` blocks at the
+    // root and trivially "passes" while exercising nothing. Nested barrels
+    // (e.g. ir/ir.zig) carry their own `test { refAllDecls }`, so this chains
+    // into them.
+    @import("std").testing.refAllDecls(@This());
+}