Remove the unused ByteBuf writers from LightProtoCodec - #34

Problem

LightProtoCodec still has ByteBuf writers that nothing calls since serialization moved to the byte[] and NIO writers, and a LITTLE_ENDIAN constant that nothing reads. The codec is a template copied into every generated package, so each copy carries this unused code.

Example

writeVarInt, writeVarInt64, writeSignedVarInt, writeSignedVarInt64, writeFixedInt32, writeFixedInt64, writeFloat, writeDouble, writeString, and the private _writeVarInt they share. Their only callers were tests:

LightProtoCodec.writeVarInt(bb, i);   // LightProtoCodecTest: encode, then check with CodedInputStream and readVarInt()

LITTLE_ENDIAN lost its last readers in #12, which replaced the Unsafe raw writers. The NIO writers added later take the byte order from the buffer itself, with nb.order().

Change

Removed the writers and the constant. LightProtoCodecTest and SegmentedByteBufTest now encode with the raw writers the generated code uses, into the array the test buffer wraps:

bb.writerIndex(LightProtoCodec.writeRawVarInt(b, 0, i));

The assertions are unchanged: protobuf-java's CodedInputStream still checks the encoding, and the LightProto readers still read it back. So those checks now cover the production writers, which previously had no direct unit tests.

Testing

mvn verify passes: 507 tests, including the 106 in LightProtoCodecTest and 7 in SegmentedByteBufTest.