Use $TMPDIR for lockdir instead of hardcoded /tmp

[davidj4tech]

The autosave lockdir prefix in scripts/continuum_save.sh is hardcoded to /tmp/. On systems where /tmp is not user-writable, mkdir "$lockdir1" fails with Permission denied, acquire_lock returns 1, and autosave stops silently — no log, no status-line indication. The only outward sign is that tmux_resurrect_*.txt snapshots stop updating, even though continuum_save.sh continues to be invoked on every status-line refresh.

Where this bites

Termux on Android. /tmp is owned by the Android shell group with mode 0771:

$ ls -ld /tmp
drwxrwx--x. 2 shell shell 40 May 17 23:49 /tmp

Non-shell users (your normal Termux user is u0_a*) cannot create entries inside it. The writable tmp lives at $TMPDIR, typically /data/data/com.termux/files/usr/tmp. Same shape applies to any sandbox or container where /tmp is mounted read-only or owned by another user.

Diagnosis trace

+ local lockdir_prefix=/tmp/tmux-continuum--lock-
...
+ mkdir /tmp/tmux-continuum--lock-17790792
mkdir: cannot create directory '/tmp/tmux-continuum--lock-17790792': Permission denied
+ return 1

This has been the behavior since the lockdir code was introduced in 800488c (2019).

Fix

Use ${TMPDIR:-/tmp} so the lockdir follows the environment's notion of a writable tmp directory. Falls back to /tmp when TMPDIR is unset, preserving existing behavior on Linux and macOS.

-	local lockdir_prefix="/tmp/tmux-continuum-$(current_tmux_server_pid)-lock-"
+	local lockdir_prefix="${TMPDIR:-/tmp}/tmux-continuum-$(current_tmux_server_pid)-lock-"

Verified

On Termux (Android, Debian proot, user u0_a2):

• Before patch: continuum_save.sh fails at mkdir, no snapshots written for days despite the script firing on every status refresh.
• After patch: mkdir succeeds in $TMPDIR, fetch_and_run_tmux_resurrect_save_script runs, fresh tmux_resurrect_*.txt appears and the last symlink updates. Subsequent runs are correctly throttled by the existing timestamp gate.

No behavioral change on systems where TMPDIR is unset or already /tmp.