fix(buzz-acp): break WIP-resume loop on placeholder checkpoints (WO #1093) - #13

$ # AC3 BEFORE (main): fixture would be resumed — resume_wip_hint has no placeholder filter
BEFORE (main resume_wip_hint, no placeholder filter):
[WIP Resume]
A prior turn left a checkpoint at `.../OUTBOX/wip/1788281363-666978601.bundle`...
would_resume=True placeholder_bytes=40
status=RED (fixture treated as resumable)

$ cargo test -p buzz-acp --test wip_placeholder_resume -- --nocapture
    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.58s
     Running tests/wip_placeholder_resume.rs (target/debug/deps/wip_placeholder_resume-a5623485e21592e5)

running 2 tests
test placeholder_fixture_archived_not_resumed ... ok
test two_consecutive_killed_idle_skips_once_with_one_notice ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s


$ cargo test -p buzz-acp wip_checkpoint -- --nocapture
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.35s
     Running unittests src/lib.rs (target/debug/deps/buzz_acp-cffac62f1d39cfb8)

running 17 tests
test wip_checkpoint::tests::test_latest_wip_bundle_none_when_directory_empty ... ok
test wip_checkpoint::tests::test_maybe_drop_wip_does_not_fire_before_five_minutes ... ok
test wip_checkpoint::tests::test_maybe_drop_wip_fires_at_five_minutes ... ok
test wip_checkpoint::tests::test_header_only_body_is_placeholder_even_when_padded_past_1kb ... ok
test wip_checkpoint::tests::test_non_idle_outcome_clears_streak ... ok
test wip_checkpoint::tests::test_maybe_drop_wip_fires_on_termination_signal_regardless_of_elapsed ... ok
test wip_checkpoint::tests::test_real_sized_bundle_still_resumes ... ok
test wip_checkpoint::tests::test_placeholder_fixture_is_archived_not_resumed ... ok
test wip_checkpoint::tests::test_split_empty_outcome_agent_exited_is_crashed ... ok
test wip_checkpoint::tests::test_split_empty_outcome_hard_timeout_is_killed_turn_cap ... ok
test wip_checkpoint::tests::test_split_empty_outcome_idle_timeout_is_killed_idle ... ok
test wip_checkpoint::tests::test_split_empty_outcome_ok_no_output_is_empty ... ok
test wip_checkpoint::tests::test_split_empty_outcome_ok_with_message_is_none ... ok
test wip_checkpoint::tests::test_wip_outbox_path_outside_branch_watcher_glob ... ok
test wip_checkpoint::tests::test_two_consecutive_killed_idle_force_skips_wip_resume_once ... ok
test wip_checkpoint::tests::test_latest_wip_bundle_picks_newest_not_alphabetical_first ... ok
test wip_checkpoint::tests::test_kill_at_25_minutes_resumes_from_bundle_within_5_minutes ... ok

test result: ok. 17 passed; 0 failed; 0 ignored; 0 measured; 714 filtered out; finished in 0.02s

$ cargo fmt -p buzz-acp -- --check
(exit 0)