Survive a crash, and a reboot after one

kill, a closed window, a reboot: no SessionEnd arrives and the state file
stays. Each case was tried rather than reasoned about, and one of the
three was broken.

A killed session was already handled -- the process is gone, so the file
and its lock are removed within the 20 s liveness tick. An interrupted
hook write left its temporary file behind forever; those are now swept
once they are five minutes old, which is late enough that a hook part-way
through writing one does not lose the update.

The reboot case was the broken one. State files outlive a reboot and pids
are handed out afresh, so "does /proc/<pid> exist" only answers "is some
process wearing that number". Verified by giving an unrelated live process
the pid of a dead session: the ghost sat in the panel as a session waiting
for input, and would have stayed there forever, asking for an answer
nobody could give. The pid is now pinned to the process start time from
/proc/<pid>/stat, recorded when the state is written and compared when it
is read.

Files written before that field existed compare only on existence, as
before, so a session open across the upgrade is not evicted.

An abandoned flock needed nothing: the kernel drops it when the holder
dies, so there is no deadlock to recover from.

tests/test-sessions.js

@@ -27,11 +27,12 @@ function check(name, condition, detail = '') {
 }
 
 const now = GLib.get_real_time() / 1e6;
-function write(id, state, cwd, agoSeconds, pid) {
+function write(id, state, cwd, agoSeconds, pid, pidStart = 0) {
     const payload = {
         session_id: id, state, cwd, since: now - agoSeconds,
-        event_ts: now, pid, event: 'test', notification_type: '',
-        message: '', zellij_session: 'ztest', zellij_pane: '1', transcript: '',
+        event_ts: now, pid, pid_start: pidStart, event: 'test',
+        notification_type: '', message: '', zellij_session: 'ztest',
+        zellij_pane: '1', transcript: '',
     };
     GLib.file_set_contents(
         GLib.build_filenamev([STATE, `${id}.json`]), JSON.stringify(payload));
@@ -50,6 +51,9 @@ write('s-dead', 'waiting', '/home/u/proj-dead', 5, deadPid);
 // Written by the older hook, which had a fourth state. Files like this
 // survive an upgrade in a session that was already open.
 write('s-legacy', 'idle', '/home/u/proj-legacy', 5, livePid);
+// Survived a reboot: the pid exists again, but belongs to something else now.
+// Without an identity check this sits in the panel forever as a live session.
+write('s-ghost', 'waiting', '/home/u/proj-ghost', 99999, livePid, 1);
 GLib.file_set_contents(GLib.build_filenamev([STATE, 'notes.txt']), 'ignored');
 GLib.file_set_contents(GLib.build_filenamev([STATE, 'half.json']), '{"broken');
 
@@ -67,6 +71,10 @@ store.connect('changed', () => {
             stateRank('waiting') < stateRank('busy'));
 
         check('dead session dropped', !s.some(x => x.sessionId === 's-dead'));
+        check('reused pid from a previous boot dropped',
+            !s.some(x => x.sessionId === 's-ghost'));
+        check('and its file removed',
+            !GLib.file_test(GLib.build_filenamev([STATE, 's-ghost.json']), GLib.FileTest.EXISTS));
         check('truncated file skipped, others survive', s.length === 5,
             `got ${s.length}: ${s.map(x => x.sessionId).join(',')}`);
         check('non-json ignored', !s.some(x => x.sessionId.includes('notes')));