Add phpix to shippedCommands so the wasmer layer builds its webc and
collects its tests into the flake checks (checks.x86_64-linux.phpix).
The tests cover the three modes end to end under wasmer: --version,
`phpix run` of a script exercising the embedded libphp (core, json,
pcre), and `phpix serve` answering real GET/POST requests from curl
over --net.
The package needs manifest entrypoint = phpix (set via passthru.wasmer
in a prior commit): that is what makes wasmer consult the command's
detected wasm features — tail calls — when picking an engine for a
package run.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Closes #25