feat(process): redesign proc_* on PhpResource process handles

proc_open/proc_close/proc_get_status/proc_terminate represented the
process handle as a PhpMixed, which cannot hold a live child process or
its pipes, so they were stubs or todo!(). Model the handle as a new
PhpResource::Process variant and child pipes as a StreamBacking::Pipe,
with a native Descriptor enum for descriptorspec; proc_open now returns
io::Result and fills pipes as IndexMap<i64, PhpResource>.

Rewire the Symfony Process pipes and the Console terminal/cursor onto
the new types, removing the "PhpMixed cannot carry a PhpResource"
todo!()s. The remaining todo!()s are genuine syscall leaves
(proc_terminate signal delivery, stream_select, stream_set_blocking,
posix_kill, pty, fd>=3) left unimplemented since no syscall crate is
introduced.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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