Add complete HPACK encoder/decoder at crates/net/src/http2/hpack.rs: - Integer encoding/decoding with prefix bits (§5.1) - Huffman coding via canonical code generation from RFC code lengths (§5.2, Appendix B) - Static table with 61 predefined headers (Appendix A) - Dynamic table with FIFO eviction and configurable max size (§2.3) - All header field representations: indexed, literal with/without indexing, never indexed (§6) - Dynamic table size updates (§6.3) - 41 tests including RFC 7541 Appendix C examples (C.2-C.6) Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>