Adds `examples/ffi-jni-call-03-methodid-sharing.sx` with two
`#jni_call` sites against the same (class, method, sig). Today each
site emits its own `GetObjectClass` + `GetMethodID` + `Call<Type>Method`
sequence (8 vtable indirections total for the two-call test); 1.17
will collapse the two `GetMethodID` calls into a single cached
`jmethodID` static slot populated at module init, mirroring the
`OBJC_SELECTOR_REFERENCES_*` shape that 1.5 introduced for `#objc_call`.
Runtime is a no-op — `unused_jni` is reachable through a
runtime-readable `g_should_call` global that stays false, so the JNI
dereferences never execute. A plain `if false` would get
constant-folded, taking the function definition out of the IR
entirely; the global keeps both the function and its body present
for the IR-snapshot harness.
IR snapshot at `tests/expected/ffi-jni-call-03-methodid-sharing.ir`
locks the pre-caching shape. The next commit (1.17) updates it to the
collapsed shape.
113/113 host tests pass.
13018ef3b4