docs: mark fiber-async / race plans SUPERSEDED by the unified context.io stack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 Prepend SUPERSEDED banners to the three historical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-FIBERS.md

@@ -1,3 +1,11 @@
+> **SUPERSEDED (2026-06-28).** The fiber-async layer described here — `sched.go` /
+> `wait` / `cancel` over `Task($R)` (B1.4a) — was RETIRED and folded into the unified
+> `context.io` async stack (`async` / `await` / `cancel` / `race`) in PLAN-IO-UNIFY
+> Phase 5. See `current/PLAN-IO-UNIFY.md` (`## Status (2026-06-28)`) for the current
+> design. The `Scheduler` ENGINE primitives this doc documents (swap_context, guarded
+> mmap stacks, spawn / run / yield_now / suspend_self / wake / sleep / block_on_fd,
+> virtual-time timers) REMAIN and are still accurate. Body below is a historical record.
+
 # PLAN-FIBERS — Stream B1 (fibers + Io + M:1 scheduler)
 
 > **STATUS: ✅ COMPLETE.** The pure-sx M:1 async runtime is feature-complete end-to-end