test(ir): lock pure JNI decision helpers before A6.2 extraction (A6.2 scaffolding step 1)

Test-first scaffolding for the JNI FFI domain (Phase A6.2) before the pure
helpers move out of lower.zig. Visibility-only change — no behavior change.

- 2 new lower.test.zig tests for the pure JNI helpers lacking unit coverage:
  - jniMangleNativeName: `/`->`_` separator, `_`->`_1` escape (path AND method),
    `Java_` prefix, `_sx_1` infix (2 cases lock all rules).
  - isJniReturnTypeSupported: void/bool/s32/s64/f32/f64 + pointer/many-pointer
    -> true; other widths (s8/s16/u8/u32/u64) + by-value struct -> false.
- JNI descriptor derivation (writeType/deriveMethod) is already extracted into
  jni_descriptor.zig (15 tests) — not part of A6.2.
- Widened jniMangleNativeName -> pub (file-scope free fn; isJniReturnTypeSupported
  already pub). Reached from the test via ir_mod.lower.*. No logic touched.
- Recorded the A6.2 coverage inventory + residual emission-bound gaps
  (synthesizeJniMainStub*/lowerJniCall/lowerJniConstructor/lowerSuperCall/
  getJniEnvTlFids stay in lower.zig; jniMapParamType is a trivial resolveType
  wrapper) in ARCH-SAFETY.md.

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

src/ir/lower.zig

@@ -16590,7 +16590,7 @@ pub fn isJniReturnTypeSupported(table: *const @import("types.zig").TypeTable, re
 /// `Java_<pkg-mangled>_<Class>_sx_1<method-mangled>`. JNI mangling:
 /// `/` → `_`, `_` → `_1`. The `sx_` prefix matches the Java-side
 /// `private native sx_<name>(...)` delegate.
-fn jniMangleNativeName(allocator: std.mem.Allocator, foreign_path: []const u8, method_name: []const u8) ![]u8 {
+pub fn jniMangleNativeName(allocator: std.mem.Allocator, foreign_path: []const u8, method_name: []const u8) ![]u8 {
     var buf = std.ArrayList(u8).empty;
     try buf.appendSlice(allocator, "Java_");
     for (foreign_path) |ch| {