[cablehead]

Adds disconnect-cleanup coverage for .cat --follow and .last --follow, the gap called out in #55.

cross-stream 0.14.1 (dff9662) fixed the follow loops to observe the caller's interrupt, so a disconnected SSE client no longer parks a thread until the next matching frame. http-nu already depends on 0.14.1 (since v0.18.0), but nothing tested the follow case.

Changes

• Extract the body of test_bus_sub_released_when_client_disconnects into a shared helper assert_streams_released_on_disconnect.
• Add test_cat_follow_released_when_client_disconnects and test_last_follow_released_when_client_disconnects, each running a store-backed server whose handler follows a topic nothing writes to.

Verification

• Both new tests pass on cross-stream 0.14.1 and fail on 0.14.0 (threads 17 -> 27, one parked per disconnected client), confirming they catch the regression.
• Live re-test (20 SSE clients, no appends): thread count returns to baseline within 0.2s for both .cat --follow and .last --follow.
• Full scripts/check.sh passes.

Closes #55.