- tap_engine.rs: Backoff::next_delay now matches spec §4.3 exactly:
250ms -> 500ms -> 1s -> 2s -> 5s cap (no intermediate 4s step from
naive doubling). Test asserts the spec enumeration including the
"stays at 5s forever" cap-after-step-5 invariant.
- session_map.rs: removed the dead 'Closing && tap.is_some()' branch
from drive_all_sessions. AppState::close already handles teardown
inline (fires close_tx, aborts task, clears field, advances
state, removes entry). The branch was never exercised and
pre-paved a "future peer-initiated close" path -- both AGENTS.md
anti-patterns ("don't pre-pave the wrong pattern").
Spec ref: 2026-06-28-slice-2-agent-tap-design.md §4.3 step 4 + §5.1 step 3.
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