refactor: remove the packet macros, WriteHeader and PacketUtils

Nothing in the server writes or reads a packet by hand any more, so the
helpers for doing so go:
- CBITSTREAM, CMSGHEADER, CINSTREAM, CINSTREAM_SKIP_HEADER, SEND_PACKET,
  SEND_PACKET_BROADCAST and HEADER_SIZE leave dCommonVars.h;
- the free BitStreamUtils::WriteHeader (LUBitStream::WriteHeader writes the
  same bytes) and the unused PacketUtils::SavePacket are deleted.
The last raw reads are replaced: WorldServer builds its input stream
directly, the master packet logs read the header with
LUBitStream::ReadHeader instead of peeking at packet->data[1] and [3], and
MessageInspector reads a sent game message's header with the new
NetGameMsg::ReadPacketHeader (the counterpart of WritePacket) instead of
memcmp/memcpy. packet->data[0] is still compared with RakNet's own
connection IDs.

The frozen oracles keep using the macros verbatim through the test-only
tests/dGameTests/LegacyPacketMacros.h; the HeaderSkip tests, which only
tested CINSTREAM_SKIP_HEADER, are removed.

docs/PacketArchitecture.md: "where we are" now describes the final state
and what still touches raw bytes (RakNet IDs, replica headers, behavior bit
streams), and a new section collects the known wire discrepancies found
during the conversion, with client addresses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/dCommonTests/CMakeLists.txt

@@ -3,7 +3,6 @@ set(DCOMMONTEST_SOURCES
 	"Amf3Tests.cpp"
 	"ToUnderlyingTests.cpp"
 	"InstanceMigrationTests.cpp"
-	"HeaderSkipTest.cpp"
 	"TestCDFeatureGatingTable.cpp"
 	"TestLDFFormat.cpp"
 	"TestNiPoint3.cpp"