Building gathers the platforms, options, tests, release gates and how the core, threads, detached sessions and 9P fit, from the old design notes checked against the code

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

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-# Tty9p: a terminal with a kernel 9P mount
-
-On Linux, `Tty9p` (`SPC n 9`) opens a terminal below this pane with the
-session mounted through the kernel's v9fs. The pane asks for your sudo
-password, mounts, and starts your shell as your normal user (with your
-supplementary groups). The shell gets `PARDES_MOUNT`, the mountpoint:
-
-```sh
-cat "$PARDES_MOUNT/index"
-cat "$PARDES_MOUNT/pane/$PARDES_PANE/body"
-echo 'Msg hello' > "$PARDES_MOUNT/exec"
-n=$(cat "$PARDES_MOUNT/pane/new")
-```
-
-The files are those of [fs.md](fs.md). The mount lives in that pane's
-private mount namespace: other panes and the editor do not see it, so a
-Look at a path under `$PARDES_MOUNT` opens nothing. Each `Tty9p` makes its
-own mount and takes one of the session's 16 Unix/TCP connection slots. It
-works in TTY, SDL and detached sessions (the session host starts the shell,
-not an attached frontend). Dump/Restore does not remake the mount.
-
-## Setup
-
-The build installs `pardes-v9fs` beside `pardes`; the host looks for it
-beside its own executable, or at `PARDES_V9FS_HELPER` (an absolute path).
-A running editor keeps the code it started with.
-
-Linux needs `9p` and its Unix transport (`9pnet_fd`); mounting needs
-`CAP_SYS_ADMIN`, which sudo supplies. The launcher runs `sudo -E` (local
-policy must allow it) and restores the caller's `PATH` after dropping
-privileges. Nothing setuid, no passwordless sudo rule and no FUSE is
-installed. The helper takes explicit mount paths and a command; it is not a
-restricted privilege broker, so do not grant it passwordless sudo.
-
-## How it works
-
-`Tty9p` starts the normal shell and queues a quoted helper command, which
-bash and fish run once their prompt is ready, as a foreground job, so sudo
-uses the terminal. A failed or cancelled authentication returns to that
-shell, as does exiting the mounted one. The unprivileged launcher makes a
-private temporary mountpoint and runs sudo; the elevated helper makes a
-private mount namespace, mounts the session's socket with
-`trans=unix,version=9p2000,cache=none,access=any,nosuid,nodev,noexec`, drops
-every root id and capability, and executes the shell. The namespace, and
-the mount, go when its last process exits. Code: `src/linux/v9fs.zig`.
-
-Linux follows `O_TRUNC` with a `Twstat` of zero length and an `mtime` hint;
-pardes takes the truncation and drops the hint.
-
-## Tests
-
-```sh
-zig build v9fs-terminal-test -Dplatform=tty   # builtin, launcher, cleanup; a sudo stand-in, no mount
-zig build v9fs-driver-test -Dplatform=tty     # the probe launcher, no privileges
-sudo -v; zig build v9fs-test -Dplatform=tty   # a real kernel mount (sudo -n); fails, not skips, without it
-```