refactor(ir): move pure JNI helpers into jni_descriptor.zig (A6.2 step 2)

Relocate the two pure JNI decision helpers out of lower.zig into
jni_descriptor.zig (already the JNI helper module), alongside the descriptor
derivation. Behavior-preserving move — no facade, since neither takes *Lowering.

- jniMangleNativeName(allocator, foreign_path, method_name) and
  isJniReturnTypeSupported(table, ret_ty) moved verbatim as pub free fns; added a
  types import + TypeId alias to jni_descriptor.zig.
- Rerouted lower.zig's 2 call sites (synthesizeJniMainStub; the JNI return-type
  guard at lower.zig:6000) through jni_descriptor.* — lower.zig already imported
  the module.
- Moved the 2 unit tests lower.test.zig -> jni_descriptor.test.zig (re-pointed to
  desc.*; a standalone TypeTable.init replaces the Module setup). Dropped the
  now-unused lower_mod alias.
- Stayed in lower.zig per PLAN A6.2 step 5/6: jniMapParamType (trivial resolveType
  wrapper), synthesizeJniMainStub(s), lowerJniCall, lowerJniConstructor,
  lowerSuperCall, getJniEnvTlFids. Java rendering stays in jni_java_emit.zig.
  Phase A6 complete.

Gate: zig build, zig build test, bash tests/run_examples.sh -> 361/0
(9 JNI .ir snapshots + 26 14xx examples green, no churn).

src/ir/lower.test.zig

@@ -765,55 +765,6 @@ test "lower: objcPropertyKind defaults + explicit ARC modifiers" {
     try std.testing.expect(lowering.objc().objcPropertyKind(.{ .name = "raw", .field_type = obj_ty, .is_property = true, .property_modifiers = &assign_mods }) == .assign);
 }
 
-// ── A6.2 scaffolding: pure JNI decision helpers ─────────────────────
-// Lock JNI native-name mangling and return-type support before they move
-// out of lower.zig (to the JNI module) in A6.2 sub-step 2.
-
-const lower_mod = @import("lower.zig");
-
-test "lower: jniMangleNativeName mangles package path + method (/ -> _, _ -> _1)" {
-    const alloc = std.testing.allocator;
-
-    // Plain path + method: `/` separators collapse to `_`, `Java_` prefix,
-    // `_sx_1` infix before the (mangled) method name.
-    const m1 = try lower_mod.jniMangleNativeName(alloc, "com/sx/App", "tick");
-    defer alloc.free(m1);
-    try std.testing.expectEqualStrings("Java_com_sx_App_sx_1tick", m1);
-
-    // Underscores in BOTH the path and the method escape to `_1` (so the JNI
-    // resolver can round-trip them), distinct from the `/`->`_` separator.
-    const m2 = try lower_mod.jniMangleNativeName(alloc, "a_b/C", "do_it");
-    defer alloc.free(m2);
-    try std.testing.expectEqualStrings("Java_a_1b_C_sx_1do_1it", m2);
-}
-
-test "lower: isJniReturnTypeSupported accepts the dispatchable set + pointers only" {
-    const alloc = std.testing.allocator;
-    var module = ir_mod.Module.init(alloc);
-    defer module.deinit();
-    const t = &module.types;
-
-    // The Call<T>Method-dispatchable primitives.
-    inline for (.{ TypeId.void, TypeId.bool, TypeId.s32, TypeId.s64, TypeId.f32, TypeId.f64 }) |ty| {
-        try std.testing.expect(lower_mod.isJniReturnTypeSupported(t, ty));
-    }
-
-    // Other primitive widths are NOT dispatchable (would hit emit_llvm's
-    // undef-producing `else` arm — the footgun this predicate guards).
-    inline for (.{ TypeId.s8, TypeId.s16, TypeId.u8, TypeId.u32, TypeId.u64 }) |ty| {
-        try std.testing.expect(!lower_mod.isJniReturnTypeSupported(t, ty));
-    }
-
-    // Pointer / many-pointer returns route through CallObjectMethod → true.
-    try std.testing.expect(lower_mod.isJniReturnTypeSupported(t, module.types.ptrTo(.void)));
-    try std.testing.expect(lower_mod.isJniReturnTypeSupported(t, module.types.manyPtrTo(.u8)));
-
-    // A pass-by-value struct return is unsupported.
-    const sname = module.types.internString("CGRectish");
-    const sty = module.types.intern(.{ .@"struct" = .{ .name = sname, .fields = &.{} } });
-    try std.testing.expect(!lower_mod.isJniReturnTypeSupported(t, sty));
-}
-
 // ── Pack projection name resolution (Feature 1, Step 2.2) ────────────
 
 const errors = @import("../errors.zig");