feat: compat.websocket server + zlib features (gated; server implies zlib)

[FarnaHerry]

Summary

Adds two optional, gated features to compat.websocket (#158), keeping the zero-dependency client build as the default:

• server — compiles the four server TUs the base build leaves out (IXWebSocketServer / IXSocketServer / IXHttpServer / IXWebSocketProxyServer). Verified against the four .cpp files: only stdlib + already-compiled IX headers, so zero new external deps. Consumers get ix::WebSocketServer (derives from SocketServer, so getPort() etc. work).
• zlib — defines = { "IXWEBSOCKET_USE_ZLIB=1" } + deps = { ["compat.zlib"] = "1.3.2" }, turning the gzip codec from its no-op into real permessage-deflate compression. The define reaches the package's own TUs; consumers need no define.

server implies zlib — a correctness trap found by reading upstream: IXWebSocketServer enables permessage-deflate by default, and the transport's extension negotiation is not gated on IXWEBSOCKET_USE_ZLIB (only the codec is). A server built without the define would advertise compression it cannot perform, so implies = { "zlib" } keeps every server build capable of what it advertises.

This branch stacks on #158 (feat/compat-websocket). Merge #158 first; then this PR's diff against main shrinks to just the feature additions.

Test coverage

New workspace member tests/examples/websocket-features (deps with features = ["server", "zlib"]), fully offline on loopback:

• Real ix::WebSocketServer round-trips (text + binary) through the compiled-in server.
• Compression is observable on the wire: a 64 KiB repeated-byte payload round-trips with message.str.size() == 65536 and wireSize == 80 (the transport reports wireSize = raw frame payload, str = decompressed content).

Negative verification: the default (no-feature) member's build has no IXWebSocketServer.o / IXSocketServer.o / IXHttpServer.o / IXWebSocketProxyServer.o, and nm over all its objects finds no server symbols — referencing the server without the feature fails at link time.

Verification (CI-pinned mcpp 2026.8.3.3 + gcc@16.1.0, MCPP_INDEX_MIRROR=GLOBAL, MCPP_BUILD_CACHE=local)

mcpp xpkg parse pkgs/c/compat.websocket.lua   →  parse OK (3 platforms / 12.0.1 / c++23 / 32 sources + 2 features)
mcpp test -p websocket                        →  test result ok. 1 passed; 0 failed   (default base, regression)
mcpp test -p websocket-features               →  test result ok. 1 passed; 0 failed   (server + zlib)

All lint scripts pass (syntax, required fields, no leading-v, mirror, package name, cross-package refs — the new compat.zlib reference resolves).

Follow-ups (not in this PR)

• wss/TLS feature (deferred, per plan — the only one that meaningfully slows CI, via compat.openssl).
• CN mirror for the tarball once mcpp-res write access exists.