Adds the JavaVM-side vtable indirection to `library/modules/platform/
android.sx` so the sx caller of `sx_query_safe_insets_jni` (1.25)
can obtain a `JNIEnv*` without the C wrapper. `#jni_call` only
dispatches through `JNIEnv*`'s vtable (a different table from
`JavaVM*`'s), so the JavaVM hop is hand-rolled here.
New decls:
- `JNI_VERSION_1_6` (0x00010006) and the `ANATIVEACTIVITY_*` byte
offsets (8, 24 on 64-bit Android — vm, clazz respectively).
- `sx_load_ptr_at(base, offset)` — load a `*void` field at a raw
byte offset. Used for both ANativeActivity fields and the JavaVM
vtable load.
- `sx_load_javavm_fn(vm, slot)` — load function pointer at the
given vtable slot. `vm` is `JavaVM*` which points to
`JNIInvokeInterface*`; the indirection is `*vm + slot * 8`.
- `sx_android_get_env(activity, out_attached)` — calls `GetEnv`
(slot 6); on `JNI_EDETACHED` falls through to
`AttachCurrentThread` (slot 4), sets `out_attached = true` so
caller can balance with `sx_android_detach_env` (slot 5).
- `sx_android_activity_clazz(activity)` — reads the jobject at byte
offset 24.
Chess Android + iOS-sim builds still clean; cross-compile 3/3
green; host 118/119. The new functions dead-strip until step 1.27
wires them into the safe-insets call site in
`android.sx::AndroidPlatform.safe_insets`.
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