v0.5.26: log to /run, tolerate tee failure

User hit `/usr/local/bin/veilor-installer: line 33: /usr/bin/tee:
input/output error` on real-hardware install. Cause: LOG was
`/var/log/veilor-installer.log`, which on the live ISO is backed by an
overlay over squashfs. A bad sector / flaky USB → tee write fails →
process substitution dies → installer aborts before the menu renders.

Two changes:

1. Move LOG to /run/veilor-installer.log — pure tmpfs, never touches
   the live medium. Same path also unaffected by /var fill or overlay
   weirdness.

2. Wrap the `exec > >(tee -a $LOG) 2>&1` redirect in a writability
   probe. If the log can't be appended to (tmpfs OOM, fd exhaustion,
   anything), skip the tee and run the installer without on-disk
   persistence rather than crashing.

Persistence is a nice-to-have for post-mortem debugging; the installer
running is the must-have. This inverts the priority correctly.

overlay/usr/local/bin/veilor-installer

@@ -17,7 +17,10 @@
 
 set -uo pipefail
 export TERM="${TERM:-linux}"
-LOG=/var/log/veilor-installer.log
+# Log to /run (pure tmpfs) — /var/log overlays squashfs on the live ISO, so a
+# bad sector on the USB medium turns `tee -a` into "input/output error" and
+# kills the installer before the menu can render.
+LOG=/run/veilor-installer.log
 
 # require_tty MUST run before the tee redirect — process substitution
 # replaces fd1 with a pipe, breaking `[[ -t 1 ]]`.
@@ -29,8 +32,12 @@ require_tty() {
 }
 require_tty
 
-# Now safe to tee output for log persistence — tty detection already passed.
+# Tee output for log persistence, but only if LOG is writable. On a flaky USB
+# the squashfs/overlay can throw I/O errors mid-write — tolerate that and
+# keep the installer running without persistence rather than aborting.
+if : >> "$LOG" 2>/dev/null; then
     exec > >(tee -a "$LOG") 2>&1
+fi
 
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