test(e2e): hold 313 to bash 3.2, which is what macOS ships

It failed on its first CI run with `mapfile: command not found`. That
constraint was already written down -- 47_cdb_prebuilt_module_path_abs.sh says
it in its own comment -- and every other user of `mapfile` in this suite
declares `# requires: gcc`, which macOS never satisfies. The rule existed and
the tests that could break it could not reach the platform that would tell
them.

Which is the same finding this file was added for, one level up: the build and
run assertions above the failure had already PASSED on macOS, so the mechanism
itself works there. Only the test did not.

Author: speak-agent

tests/e2e/313_check_stamp_on_every_platform.sh

@@ -67,11 +67,18 @@ out="$("$MCPP" run 2>&1 | tail -1)"
 # declared output goes unproduced -- it leaves the file absent and re-runs that
 # edge on every build afterwards, so a build with no stamp mechanism at all
 # stays green and merely redoes work.
-mapfile -t stamps < <(find target -name 'probe.stamp')
-(( ${#stamps[@]} == 1 )) || {
-    printf '  %s\n' "${stamps[@]}"
-    echo "FAIL: expected exactly one stamp written by the engine, found ${#stamps[@]}"
-    exit 1; }
+# ⚠️ NOT `mapfile`. macOS ships bash 3.2, which does not have it, and this file
+# runs there — `47_cdb_prebuilt_module_path_abs.sh` already says so in its own
+# comment, and every other user of `mapfile` in this suite declares
+# `# requires: gcc`, which macOS never satisfies. So the constraint was written
+# down and the tests that could violate it could not reach macOS to find out.
+# This one did, on its first CI run, with `mapfile: command not found`.
+stamp_count=$(find target -name 'probe.stamp' | wc -l | tr -d '[:space:]')
+if [ "$stamp_count" != "1" ]; then
+    find target -name 'probe.stamp' | sed 's/^/  /'
+    echo "FAIL: expected exactly one stamp written by the engine, found $stamp_count"
+    exit 1
+fi
 
 # And the other half: a satisfied edge is not re-run. Without the stamp this
 # would repeat forever, which is the symptom the exit code hides.