fix(tests): guard remaining bash 3.2 hazards from PR #95 review

Review findings #5 and #7 on PR #95 flagged two bash-3.2-incompatible
patterns despite the surrounding scripts claiming 3.2 safety:

- tests/run-bats.sh used `mapfile` (bash 4.0+), which fails immediately
  under macOS's stock bash 3.2 before any batching logic runs. Replaced
  with the `while read` loop already established in tests/run-tests.sh,
  and guarded the two downstream `${TEST_FILES[@]}` expansions with
  `${arr[@]+"${arr[@]}"}` to match that file's convention.

- `trap 'rm -rf "${CLEANUP_DIRS[@]}"' EXIT` was unguarded in
  tests/test-sync-marketplace-mirror.sh and
  tests/test-sync-plugin-content.sh: under `set -u`, if `mktemp -d`
  fails before the array is populated, the trap itself throws an
  unbound-variable error that masks the real test failure. A repo-wide
  grep for the same pattern turned up a third, unreviewed instance in
  tests/test-check-release-needed.sh. Fixed all three with the guarded
  idiom already used elsewhere in the repo.

Extended the existing bash-3.2-hazard static check (test 16 in
tests/test-vale-wrap.sh) to scan all four fixed files going forward,
so a regression of either pattern fails the suite instead of only
surfacing on a real bash 3.2 host.

Refs: PR #95
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01X7GvKuJfy2WrdBmUttV4DT

tests/run-bats.sh

@@ -16,7 +16,15 @@ if [[ ! -x "$BATS" ]]; then
     exit 1
 fi
 
-mapfile -t TEST_FILES < <(
+# Collected with a `while read` loop rather than `mapfile` — macOS ships
+# /bin/bash 3.2, which has no `mapfile`. Process substitution (not a pipe)
+# keeps the loop in this shell so the appends survive. `sort` is still fed
+# newline-delimited output, exactly as before. Same convention as
+# tests/run-tests.sh.
+TEST_FILES=()
+while IFS= read -r f; do
+    TEST_FILES+=("$f")
+done < <(
     find "$REPO_ROOT" -name "*.bats" \
         -not -path "*/tests/bats/*" \
         -not -path "*/test_helper/*" \
@@ -48,7 +56,7 @@ source "$REPO_ROOT/scripts/lib/batch-run.sh"
 
 declare -a batch_args=()
 i=0
-for f in "${TEST_FILES[@]}"; do
+for f in ${TEST_FILES[@]+"${TEST_FILES[@]}"}; do
     i=$((i + 1))
     cmd="$(printf '%q %q; echo $? >%q' "$BATS" "$f" "$SCRATCH_ROOT/$i.status")"
     batch_args+=("$i" "$cmd")
@@ -59,7 +67,7 @@ FAIL=0
 TOTAL_OK=0
 TOTAL_NOT_OK=0
 i=0
-for f in "${TEST_FILES[@]}"; do
+for f in ${TEST_FILES[@]+"${TEST_FILES[@]}"}; do
     i=$((i + 1))
     rel="${f#"$REPO_ROOT"/}"
     echo "=== $rel ==="