feat: add compat.websocket 12.0.1 (IXWebSocket RFC 6455 client, zero deps)

[FarnaHerry]

Summary

Add compat.websocket 12.0.1 — IXWebSocket, a pure C++ RFC 6455 WebSocket client, as a header-compat (Form B) package with zero external dependencies.

• Descriptor: pkgs/c/compat.websocket.lua — language="c++23", import_std=false; the 32 client TUs transcribe upstream CMake's IXWEBSOCKET_SOURCES minus the four server TUs (IXWebSocketServer / IXSocketServer / IXHttpServer / IXWebSocketProxyServer); none of the retained TUs reference any of them, so the link is clean.
• Zero deps by construction: TLS off (no IXWEBSOCKET_USE_TLS; OpenSSL/MbedTLS/AppleSSL TUs not built), zlib off (IXWEBSOCKET_USE_ZLIB unset; gzip codec compiles to a no-op). No features are offered yet — the server side and wss are deliberately out of scope for the first version.
• Mirror: plain-string GLOBAL url (no mcpp-res write access here), per docs/cn-mirror.md fallback. A maintainer can flip to { GLOBAL, CN } — sha d23bdc91… is stable.
• Test (tests/examples/websocket): fully offline and self-contained — it runs its own minimal RFC 6455 echo server on loopback raw sockets (own SHA-1 + base64 for Sec-WebSocket-Accept, own frame codec) and drives ix::WebSocket against it, asserting the handshake, client masking, text/binary echo, ping/pong both directions, fragmentation reassembly, and the close handshake.

Verification

Ran with the CI-pinned mcpp 2026.8.3.3, gcc@16.1.0, MCPP_INDEX_MIRROR=GLOBAL, MCPP_BUILD_CACHE=local, cold target//.mcpp/:

mcpp xpkg parse pkgs/c/compat.websocket.lua   →  parse OK (3 platforms / 12.0.1 / c++23 / 32 sources)
mcpp test -p websocket                        →  test result ok. 1 passed; 0 failed

11 assertions all green (ALL WEBSOCKET ASSERTIONS PASSED), including the handshake accept validation, the masked-bit check, and a fragmented-message reassembly. All lint scripts pass (syntax, required fields, no leading-v, check_mirror_urls, check_package_name, cross-package refs).

See .agents/docs/2026-08-06-add-websocket-plan.md for the full shape decision, platform flags, and the two test pitfalls found and fixed (IXWebSocket's automatic-reconnection default, and the early masked-bit assertion).

Follow-ups (not in this PR)

• CN mirror on mcpp-res/websocket once write access is available.
• A wss/TLS feature via compat.openssl / compat.mbedtls.
• A C++23 thin wrapper header (websocket.hpp) over the client.