fix: Use uint64_t for RustBuffer fields to fix 32-bit ARM ABI

RustBuffer must use uint64_t (not size_t) for capacity/len fields
to match UniFFI's C ABI on all platforms. Using size_t causes
stack corruption on 32-bit ARM (armeabi-v7a) where size_t is 4 bytes
but Rust expects 8-byte u64.

Fixes crash in constructor marshalling on Amazon Fire TV.

References:
- https://mozilla.github.io/uniffi-rs/0.27/internals/api/uniffi/struct.RustBuffer.html
- mozilla/uniffi-rs#2681

Author: sfourdrinier

cpp/includes/RustBuffer.h

@@ -11,7 +11,7 @@
 #include <jsi/jsi.h>
 
 struct RustBuffer {
-  size_t capacity;
-  size_t len;
+  uint64_t capacity;
+  uint64_t len;
   uint8_t *data;
 };