Give the pipe its own fields in Pipe

Not a pure move: state moves. Pardes's `pipe_seq` and `pipe_wait` become
Pipe.zig's own fields `seq` and `wait`, and Pardes embeds one as
`pipe: Pipe = .{}`. pipeRequest reads only the request in flight, so it now
takes `pipe: *const Pipe` and the four shells call
`core.pipe.pipeRequest(id)` (their Pipe imports go away again); the other
pipe functions still need the panes and keep `p: *Pardes`, writing
`p.pipe.seq/wait`. Field reads in dump.zig, the shells' tests and two test
files follow (`core.pipe_wait` becomes `core.pipe.wait`). No behaviour
change.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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