docs+tools: the threading fix measured 5/5 against 1/5, and a driver lock

resume_reliability.sh counts, per boot, whether the title's loader thread became
the CALLER of a kernel call. Counting boots where the press reached the title:
before the fix 1 of 5 ran the loader; after it, 5 of 5. Three of the post-fix
boots were driven far enough to classify the screen and all three reached the
MAIN MENU, each with the same shape - title at 216-252s, 40 kernel calls by the
loader, 6 ResolvePath reads. The failing boots before had zero of both, every
time.

Caveat kept in the doc: pre-fix runs were interleaved with compiles and a race is
load-sensitive, so the counts corroborate the mechanism rather than carry it.

Also fixes a self-inflicted confound. Two runs came back 'title_at_s=none' with
the emulator log ending in 'Killed', which I first read as the title detector
being too strict. It was not: an earlier copy of the script was still running
because 'pkill -f resume_reliability.sh' had matched the shell running it rather
than the script, and each of that copy's iterations kills the emulator BY NAME -
outside run-canary's lockfile. The harness now takes its own lockfile and refuses
a second copy, and the note records that pkill -f matches your own shell.

tools/re-capture/resume_reliability.sh

@@ -21,6 +21,18 @@ mkdir -p "$OUT"
 CSV="$OUT/$TAG.csv"
 echo "run,title_at_s,thread_created,thread_ran_calls,resolvepath_after,screen" > "$CSV"
 
+# Refuse to run two of these at once. Each iteration below kills the emulator by
+# name, which is outside run-canary's lockfile — so a second copy of this script
+# silently killed the first copy's boots mid-run and they were recorded as
+# failures ("title_at_s=none", the emulator log ending in "Killed"). One
+# emulator at a time has to mean one DRIVER at a time too.
+exec 8>/tmp/resume-reliability.lock
+if ! flock -n 8; then
+  echo "resume_reliability.sh: another run is in progress (lock /tmp/resume-reliability.lock)" >&2
+  echo "  kill it with:  pkill -x resume_reliability.sh   # -x, NOT -f: -f matches your own shell" >&2
+  exit 1
+fi
+
 for i in $(seq 1 "$RUNS"); do
   pkill -9 -x xenia_canary 2>/dev/null; sleep 3
   rm -f /tmp/xenia-canary.lock