fibers: address adversarial review of the B1 changes (6 findings)

UFCS generic overload resolution (issue 0157 follow-ups):
- P1-a: call planning (calls.zig) used the last-wins fn_ast_map winner
  while lowering reselected by receiver, so the planned result type
  could disagree with the dispatched function and misbox the result.
  Both now share selectUfcsGenericByReceiver(.., fd0).
- P1-b: selection scanned module_decls globally, flagging a
  transitively-hidden same-named overload as a false ambiguity. Now
  two-tier: directly-visible authors first (ambiguity only among
  those), global fallback for receiver-reachable namespaced methods
  (e.g. Task.cancel) that defers to fd0 on a hidden tie.
- P2-b: boolean specificity tied *$T with *Box($T). Now peels pointer
  layers so the structurally-narrower receiver wins.

Scheduler (sched.sx):
- P1-c: a second concurrent Task.wait overwrote the single waiter slot
  -> silent deadlock. Now one-awaiter-per-task loud abort.
- P2-c: sleep(negative) rewound the monotonic virtual clock. Rejected
  loudly.

(P2-a, non-generic-winner-hides-generic, did not reproduce -- the
non-generic arm already falls through.)

Regressions: examples/generics/0218 (receiver specificity +
plan/lowering agreement), examples/concurrency/1818 (negative-sleep
abort), 1819 (double-wait abort). Suite green 758/0.

current/CHECKPOINT-FIBERS.md

@@ -815,3 +815,16 @@ incomplete); a dedicated effort; lambda workers are the idiom meanwhile.
   runtime is feature-complete end-to-end (1800–1817), all adversarially reviewed. Suite GREEN
   **755/0**. Five compiler bugs fixed across the stream (0151/0152/0153/0154/0156-P1/0157 — 0151-3 in
   B1.2). Next carve: Stream B2 (channels / cancel / async stdlib).
+- **Post-review hardening (this session) — 6 findings from an adversarial review of the B1 commits.**
+  Fixed: **P1-a** the UFCS generic PLANNER (`calls.zig`) used the last-wins `fn_ast_map` winner while
+  lowering reselected by receiver → plan/lowering could disagree and MISBOX the result; now both share
+  `selectUfcsGenericByReceiver`. **P1-b** the selection scanned `module_decls` globally, flagging a
+  transitively-hidden same-named overload as a FALSE ambiguity; now two-tier — directly-visible authors
+  first (ambiguity only among those), global fallback for receiver-reachable namespaced methods (e.g.
+  `Task.cancel`) that defers to `fd0` on a hidden tie. **P2-b** boolean specificity tied `*$T` with
+  `*Box($T)`; now peels pointer layers so the structurally-narrower receiver wins. **P1-c** a second
+  concurrent `Task.wait` overwrote the single waiter slot → silent deadlock; now one-awaiter-per-task
+  loud abort. **P2-c** `sleep(negative)` rewound the virtual clock; now rejected loudly. (**P2-a**
+  non-generic-winner-hides-generic did not reproduce — the non-generic arm already falls through.)
+  Regressions: `examples/generics/0218` (receiver specificity + plan/lowering agreement),
+  `examples/concurrency/1818` (negative-sleep abort), `1819` (double-wait abort). Suite GREEN **758/0**.