Implements §3 Sub-phase 2.0 of the state-conditional Q synthesis spec
(docs/superpowers/specs/2026-05-28-state-conditional-q-synthesis.md).
Adds a done-gated trade-magnitude envelope that clamps the V regression
target before MSE, bounding `(v_pred - target)²` regardless of
fat-tail trade-close spikes from `returns = r + γ(1-done)·v_tp1`. The
envelope decouples from the C51 atom span (slots 484/485) so it can
tighten below the atom span floor as the trade-magnitude EMA narrows.
Wiring:
* New ISV slots 593-596: V_MAGNITUDE_EMA, V_TARGET_MAX, V_TARGET_MIN,
V_TARGET_K. RL_SLOTS_END = 597.
* New kernel `rl_v_target_envelope_update.cu` — done-gated Wiener-α
EMA of `max(|reward| | done)`; derives `V_max_eff = +k × ema,
V_min_eff = -k × ema`. Sparse-aware (skip update on no-done steps),
first-observation bootstrap, single-thread single-block.
* `v_head_bwd.cu` — clamp `r_target` to `[V_min_eff, V_max_eff]`
before computing MSE. Updates `ValueHead::backward` Rust signature
to take the ISV device pointer.
* Trainer integration: single helper `launch_rl_v_target_envelope_update`
is invoked from all three reward-pipeline paths (public
`launch_apply_reward_scale`, `step_with_lobsim_reward_and_train` —
two inline sites) — single source of truth per
`feedback_single_source_of_truth_no_duplicates`.
* Bootstrap ISV slots: V_max=+200, V_min=-200, k=3.0 (matches
dd049d9a4 baseline avg trade tail per spec). Bumps ISV constant
array from 115 → 118 entries.
V plateau LR decay (also called out in the spec) was already wired
end-to-end via `rl_lr_controller` (slots 433/434/435/438) — no
duplication needed.
Smoke validation:
* Real-data 1000-step run (`alpha_rl_train --n-backtests 128`):
max l_v = 0.6024 throughout 1000 steps (well under the spec's
5.0 ceiling).
* Synthetic Rust integration test
(`tests/v_head_stabilization_smoke.rs`): 1000 steps batch=1,
cold-start window 100 steps, post-warmup max l_v = 0.0035.
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