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/test-check-release-needed.sh

@@ -95,7 +95,7 @@ run_check() {
 }
 
 CLEANUP_DIRS=()
-trap 'rm -rf "${CLEANUP_DIRS[@]}"' EXIT
+trap 'rm -rf ${CLEANUP_DIRS[@]+"${CLEANUP_DIRS[@]}"}' EXIT
 track() { CLEANUP_DIRS+=("$1"); }
 
 # --- 1. Not targeting main: silent no-op regardless of state ---