merge(sp20): restore is_win_per_env across struct/launcher/registry after Phase 3 cherry-pick

Cherry-picking Phase 3 (a8731dda4 + 84929f419) onto Phase 2 fix tip
(13ce78385) with --strategy-option=theirs dropped is_win_per_env arg + field
+ registration the wr_ema fix added. Manual restoration:

- experience_kernels.cu: re-added is_win_per_env arg to experience_env_step
- gpu_experience_collector.rs: re-added struct field, alloc, ctor entry, launcher arg
- state_reset_registry.rs: re-added FoldReset RegistryEntry
- training_loop.rs: re-added named-reset dispatch arm
- docs/dqn-wire-up-audit.md: documented the merge restoration

Test sp20_is_win_per_env_registered_fold_reset passes. Workspace + examples
compile clean. wr_ema runtime mystery (fix wired, but production still
shows wr_ema=0) unchanged — debug per 13ce78385 scaffold pending.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/dqn-wire-up-audit.md

@@ -2,6 +2,10 @@
 
 **Status:** Populated during Plan 1 Task 6 (A.5 orphan audit). Updated on every commit per Invariant 7.
 
+## 2026-05-10 — SP20 Phase 2 fix + Phase 3 merge: restore is_win_per_env
+
+Cherry-picking Phase 3 commits onto Phase 2 fix tip with `--strategy-option=theirs` dropped the `is_win_per_env` kernel arg + struct field + reset registration that the wr_ema fix branch added (Phase 3 had its own version of the `experience_env_step` signature without that arg). Manual restoration: kernel signature, struct field, alloc, launcher arg, registry entry, reset dispatch arm. Test `sp20_is_win_per_env_registered_fold_reset` passes.
+
 ## 2026-05-10 — SP20 Phase 3 Task 3.4: replay-buffer schema for per-bar aux_conf (atomic)
 
 Lands the producer→ring→consumer schema plumbing for per-bar