feat(wsl): advertise the binary path and hold the probe cache [1]

[AbysmalBiscuit]

Two independent fixes to the WSL boundary, both about a shell inside a distro
being able to talk back to the instance that spawned it.

fix(wsl): keep the last probe when the helper is briefly down

The probe poller collapsed three different outcomes into the same None:
a genuine "no foreground TUI", a missing client, and a transport error.
Any transient helper outage therefore blanked every session's cached
foreground command name, and while the helper sat in its respawn cooldown
the cache stayed blank — disabling FocusLeft/FocusRight passthrough for
a still-running nvim or tmux until the helper came back on its own.

The cache is now overwritten only on a definitive reply. A missing client or
a probe error leaves the last-known value in place. client() is still
called every tick, so a cooled-down helper keeps getting nudged back up.

feat(ipc): advertise the binary path to WSL shells

A shell inside a distro could already find a running instance through
ALACRITREE_SOCKET, but had no way to invoke the CLI: the Windows image is
reachable over interop yet is not on the distro's $PATH, so callers had to
hardcode a path or guess at one.

The running image is now exported as ALACRITREE_EXE and listed in WSLENV
with /p — the flag that has WSL rewrite the value into the distro's view of
the drive, honouring whatever automount root that distro uses instead of
assuming /mnt. It is the only one of the three exported variables that
carries a conversion flag; a pipe name and a session id are not paths. A user
who already lists ALACRITREE_EXE themselves keeps their own spelling.

Testing

cargo test -p alacritree — 477 pass. New coverage:

• ipc::tests::wslenv_lists_the_exe_path_for_conversion — the /p flag is
  applied, and an existing user entry is left alone.
• ipc::tests::a_client_reaches_the_listener_it_advertises now also asserts
  ALACRITREE_EXE matches current_exe().

The existing wslenv_gains_* tests were updated for the third variable.