docs(roadmap): record the chunk-2 session candidates — no-io idle sleep gap, cross-thread unpark

The terminal-wake fix cannot reach the (deadline, no-io) idle branch:
thread::sleep, no pipe to write. Documented as a Later item with the
two candidate mechanisms (clamped sleep vs condvar/futex), plus the
cross-thread-unpark observation from the same session.

ROADMAP.md

@@ -113,6 +113,26 @@ shared instead" check. In scope only if the data demands it.
 
 ## Later
 
+### Unwakeable idle sleep when io is absent (terminal-wake residual)
+The `(Some(deadline), None)` idle branch — timers pending, io subsystem never
+initialized — blocks in `thread::sleep` with no wake mechanism at all. The
+terminal wake (writes the wake pipe at AllDone) cannot reach it: no io, no
+pipe. Same stall as the fixed bug, in any no-io runtime: a sibling that
+blocked on an orphaned deadline sleeps it out in full after everything else
+finished. Candidates, mutually exclusive: (a) clamp the sleep (cheap, but
+turns idle into periodic wakeups), or (b) park the branch on a condvar/futex
+the AllDone path signals — and at that point consider making the condvar the
+idle primitive for the no-io runtime generally (a cross-thread unpark could
+signal it too, see below). Decide before any no-io deployment.
+
+### Cross-thread unpark
+`RuntimeInner::enqueue` does not wake idle sibling schedulers — only io
+completions write the wake pipe. Mid-flight this is masked (the enqueuing
+thread is awake and eats the work itself), but it costs parallelism: work
+enqueued by a busy thread waits until the sibling's idle poll times out.
+Candidate from the urus chunk-2 session; needs bench evidence (does the
+shared-queue handoff latency actually show up?) before a mechanism is picked.
+
 ### Per-switch cost (context shims, epoch protocol)
 The shootout's residual: per-wake latency is 0.16–0.18 µs at N=1 and
 0.8–1.2 µs at N=8+, dominated by the context-switch shims and the epoch