[fdcastel]

Addresses item 4 of #74 (feedback: "Probably should be getAlignedLength. But not only that, and not only there, buffer start should be aligned as well.").

Problem

• RowSet placed rows getMessageLength() bytes apart, so every row after the first could start misaligned for its fields.
• Every message buffer (Statement input/output, RequestMessage, RowSet) was a plain std::vector<std::byte>, so its start alignment depended entirely on the global allocator.

Changes

• New MessageBuffer.h:
  • fbcpp::MessageBuffer is a std::vector<std::byte> with an aligned allocator (impl::AlignedAllocator, which uses operator new with std::align_val_t).
  • Its start is aligned to MESSAGE_BUFFER_ALIGNMENT (16 bytes), which covers any field of a Firebird message.
• Statement: inMessage and outMessage are MessageBuffer, and so are the return types of getInputMessage() and getOutputMessage().
• RequestMessage and impl::ParameterSetter: use MessageBuffer for the message.
• RowSet:
  • The buffer is a MessageBuffer.
  • Rows are spaced by the new getRowStride(), which is the message length rounded up to MESSAGE_BUFFER_ALIGNMENT. getRawRow() still returns getMessageLength() bytes, and getRawBuffer() now holds getCount() * getRowStride() bytes.

Notes for review

• Why not getAlignedLength(): IMessageMetadata::getAlignedLength() and getAlignment() only exist from interface version 4 (Firebird 4.0 clients), and Blobs cannot be created with a Firebird 3 client library #71 showed that Firebird 3 client libraries still matter. Rounding up to the fixed 16-byte alignment is always at least as strict, costs at most 15 bytes per row, and works with any client. If you'd rather use getAlignedLength() when the client supports it, I can change it.
• Source compatibility: the return types of getInputMessage(), getOutputMessage() and RowSet::getRawBuffer() change from std::vector<std::byte> to MessageBuffer. Code that uses auto&, .data(), .size() or iterators compiles unchanged. Code that names std::vector<std::byte>& explicitly needs to change it to fbcpp::MessageBuffer&.

Tests

• RowSetSuite/rowsAreAligned uses a result set whose message length isn't a multiple of 16. It checks that:
  • the statement's input and output buffers are aligned;
  • every RowSet row starts aligned;
  • the stride is a multiple of 16 and at least the message length;
  • the SMALLINT, BIGINT and DOUBLE values read back correctly.
• The existing RowSet size checks now use getRowStride().

Full suite: 349 test cases pass (Windows, MSVC, Firebird 5.0.3 server).