rust: fix 64-bit scalar wire-format alignment on 32-bit targets in Push::alignment (#9465)

In the FlatBuffers binary wire format specification, every scalar field is
aligned to its own byte size (`1`, `2`, `4`, or `8` bytes), regardless of the
host CPU architecture. Both the `flatc` schema compiler (when computing table
`max_align` and struct field offsets) and the C++ `FlatBufferBuilder::Align` /
`PushElement` implementation align 64-bit scalars (`u64`, `i64`, `f64`) to 8
bytes on all architectures.

However, in the Rust `flatbuffers` crate (`rust/flatbuffers/src/push.rs`), the
`Push` trait provides a default implementation:

```rust
fn alignment() -> PushAlignment {
    PushAlignment::new(align_of::<Self::Output>())
}
```

And `impl_push_for_endian_scalar!` did not override `Push::alignment()`,
inheriting `core::mem::align_of::<T>()` for all primitive scalar types (`bool`,
`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `f32`, `f64`).

On 64-bit targets (`x86_64`, `aarch64`) and 32-bit ARM (`armv7`),
`align_of::<u64>() == 8`, so `align_of::<T>()` happens to equal `size_of::<T>()`.
On 32-bit x86 (`i686`, System V i386 ABI), however, the host ABI alignment of
64-bit scalars (`u64`, `i64`, `f64`) is **4 bytes** (`align_of::<u64>() == 4`),
while their FlatBuffer wire-format size and required wire alignment is **8
bytes** (`size_of::<u64>() == 8`).

As a consequence, on 32-bit x86 (`i686`):
1. `FlatBufferBuilder::push::<u64 / i64 / f64>` only aligns the builder's cursor
   and tracks `min_align` to 4 bytes instead of 8 bytes when writing 64-bit
   table fields or vectors of 64-bit scalars.
2. When a table containing a 64-bit scalar field (`uint64`, `int64`, `double`)
   is serialized on `i686`, the field can be placed at a buffer offset that is
   4-byte aligned but not 8-byte aligned.
3. When the resulting FlatBuffer is verified by C++ `flatbuffers::Verifier`
   (which checks `VerifyField<uint64_t>` with `sizeof(uint64_t) == 8` alignment
   when `FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE` / alignment checks are active)
   or transferred to a 64-bit reader, verification fails or unaligned 64-bit
   reads occur.

Fix `impl_push_for_endian_scalar!` to explicitly implement `Push::alignment()`
using `size_of::<$ty>()` so that all endian scalar types always use their
wire-format byte size as their FlatBuffer alignment across all target
architectures.

rust/flatbuffers/src/push.rs

@@ -83,6 +83,11 @@ macro_rules! impl_push_for_endian_scalar {
             unsafe fn push(&self, dst: &mut [u8], _written_len: usize) {
                 emplace_scalar::<$ty>(dst, *self);
             }
+
+            #[inline]
+            fn alignment() -> PushAlignment {
+                PushAlignment::new(size_of::<$ty>())
+            }
         }
     };
 }
@@ -98,3 +103,23 @@ impl_push_for_endian_scalar!(u64);
 impl_push_for_endian_scalar!(i64);
 impl_push_for_endian_scalar!(f32);
 impl_push_for_endian_scalar!(f64);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn endian_scalar_push_alignment_matches_wire_size() {
+        assert_eq!(<bool as Push>::alignment().value(), 1);
+        assert_eq!(<u8 as Push>::alignment().value(), 1);
+        assert_eq!(<i8 as Push>::alignment().value(), 1);
+        assert_eq!(<u16 as Push>::alignment().value(), 2);
+        assert_eq!(<i16 as Push>::alignment().value(), 2);
+        assert_eq!(<u32 as Push>::alignment().value(), 4);
+        assert_eq!(<i32 as Push>::alignment().value(), 4);
+        assert_eq!(<f32 as Push>::alignment().value(), 4);
+        assert_eq!(<u64 as Push>::alignment().value(), 8);
+        assert_eq!(<i64 as Push>::alignment().value(), 8);
+        assert_eq!(<f64 as Push>::alignment().value(), 8);
+    }
+}