Finish the portability sweep: one firmware resolver, no author paths

The probe scripts, run.sh, trace_run.sh, webui.py and two emulator tests each named the same hardcoded firmware from a source tree that is not in this repository. They now resolve QEMU and the firmware the way tools/uvk5_testenv.py does -- environment, then PATH, then whatever the checkout has -- and skip with a reason when there is nothing.

webui.py's --qemu and --elf lost their author defaults too: a bare qemu-system-arm through PATH, and no firmware until one is uploaded, which the page already reports.

tools/pc.sh

@@ -6,7 +6,9 @@
 # (different PC, or r1 reset for a new call).
 set -uo pipefail
 
-ELF="${ELF:-$HOME/uvk5-port/uvk5-sat/build/CW/nr7y.cw.elf}"
+# shellcheck source=tools/uvk5_elf.sh
+. "$(dirname "$0")/uvk5_elf.sh"
+ELF=$(uvk5_find_elf) || { echo "SKIP  no firmware found; set ELF or put one in assets/firmware" >&2; exit 0; }
 SAMPLES="${1:-6}"
 GAP="${2:-2}"
 SCRIPT=$(mktemp --suffix=.gdb)