feat: HTTP/2 Handler Implementation (TASK-204)

[copyleftdev]

Summary

Implements HTTP/2 protocol handler following the ProtocolHandler interface pattern, completing TASK-204.

• Frame serialization: SETTINGS, HEADERS, DATA, PING, WINDOW_UPDATE, GOAWAY, RST_STREAM
• HPACK compression: Static table encoder/decoder (no dynamic table or Huffman for v1)
• Stream multiplexing: 10 concurrent streams per connection
• Flow control: Connection-level and stream-level window management
• Connection lifecycle: Preface + SETTINGS exchange, GOAWAY for graceful shutdown
• Server push rejection: Automatically sends RST_STREAM with CANCEL

Files Changed

File	Description
src/http2_handler.zig	New - ProtocolHandler implementation
src/http2_hpack.zig	New - HPACK encoder/decoder with static table
src/http2_frame.zig	Added 9 frame serialization functions
src/z6.zig	Exports for new HTTP/2 types
build.zig	Test targets for new modules
tests/unit/http2_handler_test.zig	New - 13 handler tests
tests/unit/http2_hpack_test.zig	New - 16 HPACK tests
tests/unit/http2_frame_test.zig	Added 11 serialization tests

Test plan

• All 30 HTTP/2 frame tests pass
• All 16 HPACK tests pass
• All 13 handler tests pass
• Total: 59 tests passing

Deferred to future PRs

• ALPN/TLS integration
• Dynamic HPACK table
• Huffman encoding
• CONTINUATION frames
• Advanced stream prioritization

Closes #63

🤖 Generated with Claude Code