Enable PacketBuilder in no_std (alloc/slice) while preserving std API

[xyzzyz]

PacketBuilder serialization is split from std::io::Write so packet building works in no_std environments while keeping std behavior intact.

Key changes

• Introduce an internal writer abstraction (CoreWrite) and internal WriteError<IO, Content> in src/writer.rs.
• Add an explicit alloc feature and make std depend on alloc.
• Make PacketBuilder available without std.
• Keep std write(...) -> BuildWriteError API, with BuildWriteError remaining std-only.
• Add no_std-capable write APIs:
  • write_to_vec (alloc) with BuildVecWriteError
  • write_to_slice (no alloc) with BuildSliceWriteError
• Refactor IPv4/IPv6 extension writing through shared internal methods used by both std and no_std paths.
• Add docsrs cfg annotations for alloc-only APIs.
• Add focused tests for vec and slice write paths:
  • write to empty vec
  • write to pre-populated vec
  • write to slice success path
  • write to slice too-small error path

Compatibility

• std users keep existing PacketBuilder::write behavior.
• no_std users gain packet serialization through vec/slice writers.