feat(sp16-p2): adaptive Hold cost scale via ISV[461]

Per train-multi-seed-pfh9n post-mortem: observed_hold_rate climbed 0.25 → 0.52
across training while cost penalty (~0.006) was 100× smaller than per-bar
reward magnitudes (popart=0.97, cf=0.65). Hold action was effectively free,
allowing Q(Hold) to dominate via structural low-variance bias.

Fix: scale Hold cost adaptively. ISV[HOLD_COST_SCALE_INDEX=461] tracks:
  scale = clamp(1.0 + 24.0 × max(0, observed - target) / max(target, 0.01), 1.0, 25.0)

Effective cost at 100% overrun (observed=2× target): 0.006 × 25 = 0.15,
competitive with per-bar reward magnitudes (~0.01-0.1).
At/below target: scale = 1.0 (no extra penalty).

Pearl-A bootstrap + Welford slow EMA (α=0.05). Mirrors T1's
MIN_HOLD_TEMPERATURE pattern (same input signals: ISV[382] observed,
ISV[381] target).

Producer: hold_cost_scale_update_kernel.cu — single-thread cold-path,
per-epoch boundary, AFTER MIN_HOLD_TEMPERATURE in training_loop.rs.

Consumer migration (atomic per feedback_no_partial_refactor):
3 sites in experience_kernels.cu — segment_complete branch (line ~3089),
per-bar positioned-Hold branch (line ~3553), per-bar flat-Hold branch
(line ~3617). Cold-start fallback: scale=1.0 when slot ≤ 0 or
out-of-bounds (bit-identical pre-Phase-2 cost magnitude).

ISV_TOTAL_DIM: 461 → 462.

Behavioral tests (5/5 PASS on RTX 3050):
- sp16_phase2_hold_cost_scale_climbs_with_overrun
- sp16_phase2_hold_cost_scale_at_target_is_one
- sp16_phase2_hold_cost_scale_under_target_is_one
- sp16_phase2_hold_cost_scale_bounds_clamp
- sp16_phase2_hold_cost_scale_pearl_a_bootstrap

Regression: SP14 oracle suite 30/30 PASS, SP15 phase 1 oracle suite
36/36 PASS.

Instrumentation: HEALTH_DIAG[N]: hold_cost_scale_diag obs/tgt/norm/scale.

Per feedback_isv_for_adaptive_bounds + feedback_no_partial_refactor.

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

crates/ml/build.rs

@@ -867,6 +867,18 @@ fn main() {
         // see docs/dqn-wire-up-audit.md § "SP16 Phase 1 (revised)" for
         // rationale.
         "min_hold_temperature_update_kernel.cu",
+        // hold_cost_scale_update_kernel.cu — SP16 Phase 2 adaptive Hold
+        // cost scale producer. Reads ISV[HOLD_RATE_OBSERVED_EMA_INDEX=382]
+        // and ISV[HOLD_RATE_TARGET_INDEX=381] (same input chain as SP16-P1
+        // MIN_HOLD_TEMPERATURE), maps overrun
+        // `clamp(max(0, observed - target) / max(target, 0.01), 0, 1)`
+        // to a [SCALE_MIN=1, SCALE_MAX=25] multiplier, blends via Pearl-A
+        // bootstrap + Welford EMA α=0.05, writes to
+        // ISV[HOLD_COST_SCALE_INDEX=461]. Consumer: per-bar Hold cost
+        // subtraction sites in experience_kernels.cu multiply
+        // `isv[ISV_HOLD_COST_IDX=380]` by `isv[461]`. See
+        // docs/dqn-wire-up-audit.md § "SP16 Phase 2" for rationale.
+        "hold_cost_scale_update_kernel.cu",
         // SP14 Layer C Phase C.6 (2026-05-08): h_s2_aux RMS EMA producer.
         // Single-block 256-thread kernel computing RMS = sqrt(mean(x²))
         // over `h_s2_aux [B, SH2]` (aux trunk final output, no activation)