fix(wayland): a process table glass cannot read is not an absent Xwayland

[xxx]

Fixes #381.

session_display returned None for four different reasons and recover_if_due read all of them as the one that is expected — a native Wayland app with no X11 side. An unreadable /proc, or an unreadable environ/cmdline on the session's own Xwayland, switched lost-window recovery off for the life of the session with nothing said, after which list_windows reported the compositor's shorter list as fact.

The scans now separate "looked and found nothing" from "could not look". Ok(None) is the native Wayland session and nothing else; an Err is reported like a failed XProbe::connect. A read failure before the runtime-dir match stays a silent skip — that is what another user's Xwayland, or one that exited mid-scan, correctly looks like.

Recovery::warned records which step failed rather than that something did, so a /proc failure no longer consumes the line a later connect failure would have said (the same asymmetry, one branch over).

session_processes swallowed the same read_dir("/proc") failure, and doctor's leak accounting read the empty list as "the probe left nothing running" — deleting the probe's runtime dir out from under whatever still held sockets in it. It now reports that it could not tell, and keeps the dir. A failed scan is retried within the existing 500 ms LEAK_GRACE, since a host out of file descriptors is the transient that wait is there for.

Verification

• 189 glass-wayland unit tests; 93 workspace test binaries; 0 failures.
• cargo clippy --workspace --all-targets -- -D warnings and cargo fmt --check clean.
• The #[ignore]d real-session tests pass against a live sway/Xwayland (--include-ignored).
• The process table is a parameter to the scans, so the branches that decide whether recovery runs are covered against a built one: no /proc, an entry whose files cannot be read, an Xwayland with no :N argument, one serving another session, and a table whose only entry is unreadable.
• Each new test was shown to fail by reinstating the swallow it guards (5 ablations).