docs: mark fiber-async / race plans SUPERSEDED by the unified context.io stack

Phase 5 of PLAN-IO-UNIFY retired the bespoke fiber-task API (Task/go/wait/cancel)
and re-homed race onto *Future. Prepend SUPERSEDED banners to the three historical
planning docs (PLAN-FIBERS, CHECKPOINT-FIBERS, PLAN-RACE) pointing readers to
PLAN-IO-UNIFY for the current design, noting that the Scheduler engine primitives
+ the race type-machinery they document remain accurate. Bodies kept as the
historical record. Completes the plan's 'update roadmap/checkpoints' item.

Verified by parallel agents: a completeness pass (no retired API in compilable
code; sched.sx surface fully retired; migrated examples meaningful; readme on the
unified API) and an adversarial memory-safety pass over the heap reclamation (no
UAF/double-free; await/worker orderings, cancel timings, 8-way double-free+deinit,
blocking impl all reclaim cleanly with exact values).

current/PLAN-RACE.md

@@ -1,3 +1,10 @@
+> **SUPERSEDED (2026-06-28).** The `race` LOGIC described here shipped, then was
+> RE-HOMED onto `*Future` + the `Io` protocol as `context.io.race` in PLAN-IO-UNIFY
+> Phase 4. The Task-based `sched.race` over `*Task` documented below is RETIRED. See
+> `current/PLAN-IO-UNIFY.md` (`## Status (2026-06-28)`) for the current design. The
+> type-level machinery this doc documents (`RaceResult` / `make_variant` / tuple
+> reflection) is UNCHANGED and still in use. Body below is a historical record.
+
 # PLAN-RACE — Stream B2/A1: `race` over the M:1 fiber scheduler
 
 Carved from the async roadmap ([../design/execution-evolution-roadmap.md](../design/execution-evolution-roadmap.md)