[C++][Feather] Avoid reading outside sliced bitmap buffers in V1 writer

## Describe the bug

The Feather V1 writer shifts bitmap bytes when serializing a sliced Boolean
array or validity bitmap. On the final output byte, the shift loop
unconditionally reads the next source byte even when the logical slice ends in
the current byte.

A valid `ArrayData` backed by an exact-sized `Buffer` therefore causes a
one-byte heap-buffer-overflow read. The adjacent bit is also folded into the
unused bits of the serialized bitmap byte.

## Component

C++ IPC / Feather V1 writer.

## Version

Reproduced on current `main` at
`35c5ffd12173284406e4a2c86405415444e596d7`.

## Minimal reproduction

```cpp
auto owner = std::make_unique(1);
owner[0] = 0x03;
auto values = std::make_shared(owner.get(), 1);

auto data = arrow::ArrayData::Make(arrow::boolean(), 1, {nullptr, values},
/*null_count=*/0, /*offset=*/1);
auto array = arrow::MakeArray(data);
auto table = arrow::Table::Make(
arrow::schema({arrow::field("flag", arrow::boolean())}),
{std::make_shared(array)});

ARROW_ASSIGN_OR_RAISE(auto sink, arrow::io::BufferOutputStream::Create());
auto properties = arrow::ipc::feather::WriteProperties::DefaultsV1();
ARROW_RETURN_NOT_OK(
arrow::ipc::feather::WriteTable(*table, sink.get(), properties));
```

Under ASan, the write deterministically reports a one-byte
heap-buffer-overflow read in `WritePaddedWithOffset` in
`cpp/src/arrow/ipc/feather.cc`.

The same harness with `offset = 0` completes without a sanitizer error.

## Expected behavior

The writer should read only the bitmap bytes required by the logical slice and
clear unused trailing bits in the serialized bitmap byte.