chore(sp18): archaeology cleanup — strip post-deletion markers + ISV_TOTAL_DIM giant docstring

Pure-comment follow-up to SP18 Phase 1 atomic deletion (3c318953a) and
Phase 2 device-fn introduction (8da8e2e58). Strips the "DELETED by SP18
Phase 1" markers + "RETIRED" docstrings + per-slot SP3→SP18 archaeology
that survived the focused deletion diff. Per feedback_no_legacy_aliases
("avoid backwards-compatibility hacks like removed-comment markers")
and CLAUDE.md ("don't add comments that explain what the code does"),
git history is the historical record — code shouldn't narrate its own
deletion.

Three categories cleaned:

1. ISV_TOTAL_DIM giant docstring in gpu_dqn_trainer.rs (Category 1):
   collapsed ~5000-char slot-by-slot SP3→SP18 history block into a
   13-line conceptual definition pointing readers at sp14_isv_slots.rs
   for the actual range definitions. Layout fingerprint seed
   (RESERVED_GAP_461_TO_468=SP18_P1_RETIRED;) is load-bearing and
   preserved.

2. RETIRED markers in sp14_isv_slots.rs + state_layout.cuh
   (Category 2): deleted "SP16 Phase 2 RETIRED 2026-05-09" header
   blocks, "HCS_* slots [462..468) RETIRED" markers, "SP13 [380..383)/
   SP16 [461..474) ALLOCATED but RETIRED per PP.4" prose, and the
   test-removal note before the surviving MHT block lock test.
   Tightened the SP16 T3 Wiener-α comment block to describe only the
   surviving MIN_HOLD_TEMPERATURE producer.

3. Stale doc-refs across the SP18 audit consumer set (Category 3):
   updated gpu_experience_collector.rs prose, sp5_isv_slots.rs SP13 v3
   description, gpu_dqn_trainer.rs slot-380 constructor block,
   state_reset_registry.rs HRC_*/MHT_*/TDB_* "mirrors slot N" refs +
   the registry-block introduction comment + the deleted-test marker
   at end of file, training_loop.rs launcher-call/diag-emit/
   dispatch-arms "DELETED" markers, build.rs cubin manifest "DELETED"
   marker, gpu_aux_trunk.rs trailing "HoldCostScaleUpdateOps DELETED"
   block, and tests/sp14_oracle_tests.rs leading "tests DELETED" marker.

Production-source grep verification (post-cleanup): zero matches for
HOLD_COST_SCALE_INDEX | HCS_TARGET | HCS_DIFF | HCS_PREV | HCS_SAMPLE
| HOLD_COST_CONTROLLER_GAIN | HOLD_COST_FLOOR_RATIO |
HOLD_COST_CEIL_RATIO | hold_cost_scale_update across crates/ml/src/
*.rs/*.cu/*.cuh.

Audit doc + script preserved per design:
- scripts/audit_sp18_consumers.sh grep patterns retain HOLD_COST_SCALE
  / HCS_* — those are the diagnostic tool.
- docs/sp18-wireup-audit.md historical sections retain references
  (audit doc IS the design-history record); fingerprint section
  regenerated to reflect post-cleanup hit counts (audit --check passes).
- docs/superpowers/specs/* + docs/superpowers/plans/* unchanged
  (spec/plan files retain HOLD_COST_SCALE references unchanged per
  feedback_trust_code_not_docs corollary — those are the past
  design-decision record).
- docs/dqn-wire-up-audit.md current-state header section gains a
  2026-05-09 archaeology-cleanup-pass entry citing the affected
  files + verification (per Pre-commit Invariant 7).

Verification:
- cargo check --workspace: clean (warnings only — pre-existing).
- cargo test -p ml --lib --no-run: compiles clean.
- cargo test --doc -p ml: 21 failures pre-existing (verified via stash;
  identical 21 failures on parent commit 8da8e2e58 — unrelated to
  this cleanup).
- bash scripts/audit_sp18_consumers.sh --check: passes.

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

crates/ml/tests/sp14_oracle_tests.rs

@@ -2170,11 +2170,6 @@ mod sp16_phase1_min_hold_temperature_gpu {
     }
 }
 
-// SP16 Phase 2 / Phase 3 oracle tests (11 GPU tests + helpers + 1
-// hardcoded-alpha regression test) DELETED by SP18 Phase 1
-// (2026-05-09). The hold_cost_scale_update_kernel cubin they
-// include_bytes! is gone. See docs/sp18-wireup-audit.md (A2).
-
 // ═══════════════════════════════════════════════════════════════════════════
 // SP16 Phase 0 (2026-05-08) — Q-by-action HEALTH_DIAG diagnostic.
 //