roadmap: remove RFC 004 spinning sections (3. Spinning workers, 4. RFC 004 bench + interaction pass)

The spinning experiment is excised from master. Drops the two dedicated v0.9
sections describing it. Inline RFC-004 mentions in the v0.9 intro and the
queue-topology decision record are intentionally left for a manual short-term
-plan pass.

ROADMAP.md

@@ -88,21 +88,6 @@ it's a Config knob, not a feature rebuild). Same sweep as the rq shootout.
 Acceptance flips the default on and re-baselines for RFC 004.
 This is done, see results annotated in RFC005.
 
-### 3. Spinning workers (RFC 004)
-Bounded spin-before-park for idle schedulers, killing the ~100 µs
-`thread::sleep` worst case on cross-thread handoff. Two Config knobs:
-`spin_budget_cycles` (0 recovers today's behaviour) and `max_spinners`
-(default N/2); one new atomic `n_spinning`. No run-queue changes — lands on
-the frozen `rq-mutex` substrate, independent of the slot mechanically.
-Credit for the idea: Dennis Gustafsson - [Parallelizing the physics solver BSC 2025](https://www.youtube.com/watch?v=Kvsvd67XUKw). 
-
-### 4. RFC 004 bench + interaction pass
-Re-run the sweep with spinning enabled, slot on and off. The known
-interaction: once idle pickup latency ≪ slice, the slot's latency trade
-(occupant waits out the waker's slice while other schedulers idle) may stop
-paying — RFC 005's documented escape hatch is Go's "spinners exist → push
-shared instead" check. In scope only if the data demands it.
-
 ---
 
 ## Later