feat: rollout_ablate_frac exploration floor vs hack-saturation (route/route2)

Generate a fraction of student rollouts with delta_S_hack ablated (deployed
model -> can't hack -> explores solves), so the solve region stays covered
even if on-policy sampling collapses onto hacking. Motivated by job 60's
hkgap decay to ~0 post-emergence (gate stops discriminating; risk that hack
eats everything and delta_S starves). Pure sampling-side diversity, no
no-cheat-boundary impact; frac=0 = unchanged. Smoked at frac=0.5.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

RESEARCH_JOURNAL.md

@@ -2302,3 +2302,62 @@ arm) so the 5-arm overlay reads uniform numbers.
 5-arm clean sweep queued (40-44, all #164). On completion: confirm the deploy-solve>=
 train-solve gap reproduces per-arm, and read run_tests deploy-solve specifically (the
 watch-item for whether deploy-mode rollouts are needed).
+
+# 2026-06-01
+
+## Exploration floor against hack-saturation: `rollout_ablate_frac` (route/route2)
+
+**Context.** Live audit of job 60 (route2, scale-matched `delta_S_hack` quarantine,
+seed 41) past the step-10 emergence. The discrimination gauge `hkgap`
+(`ema_hack_cos - ema_clean_cos`) started clearly positive (+0.09 @ step 2) and
+decayed through zero to weakly negative (~-0.03 across steps 27-41); `tau` rode it
+down to ~0. So the calibrated gate is faithfully tracking a signal that has gone
+dead, the student-side `cos>tau` route is back to a coin flip, and routing is
+carried mostly by the forced teacher-anchor routing (`qE~0.55`). Solving survived
+(clean rollouts still kept), but this exposed the structural risk the user named:
+on-policy sampling can collapse onto hacking, at which point every rollout routes to
+the deleted quarantine and the deployed `delta_S` never sees a solve gradient. Hack
+eats everything.
+
+**Decision.** Add a standard RL exploration floor: generate a fraction
+(`rollout_ablate_frac`, default 0) of the student rollouts with the quarantine
+ablated, i.e. from the deployed model, which cannot express the hack and so explores
+the solve region. This guarantees solve-region coverage regardless of how saturated
+the full policy gets. Pure sampling-side diversity, no new loss, no reward change, no
+grader: it does not touch the no-cheat boundary. It accepts a slight off-policy
+mismatch (GRPO already tolerates off-policy samples via clipping/reuse), which the
+user judged worth it for the coverage. This is the previously-deferred "deploy-mode
+rollouts" idea (see prior entry), promoted from deferred now that job 60 shows the
+saturation pathway is live.
+
+Bonus property for our setup: at deploy `delta_S_hack` is zeroed, so the deployed
+model *is* the ablated model. Generating a fraction ablated trains `delta_S` partly
+on the exact distribution it faces at deploy, closing the train/deploy gap, not just
+preventing starvation.
+
+**Subtlety corrected mid-design (load-bearing).** Generation policy and gradient
+policy are decoupled in GRPO: the gradient comes from the teacher-forcing recompute,
+not the sampling pass. So generating ablated does *not* by itself keep `delta_S_hack`
+gradient-free; a solve rollout that happens to contain hack-ward tokens would still
+backprop into the quarantine under a full-model recompute. We do NOT match the
+recompute ablation per-subset (would need two backwards). We rely instead on the fact
+that a genuine-solve rollout is clean-ward, so it is not flagged, so route2 leaves its
+full gradient in `delta_S` anyway. The exploration value (coverage) is what we are
+buying; the gradient routing is unchanged.
+
+**Implementation.** `train.py`: `Config.rollout_ablate_frac`; a `gen_students(enc, n)`
+helper that splits the n student rollouts into `round(n*frac)` ablated (under
+`ablate_quarantine`) + the rest full, pads, concatenates. Both generate call sites
+(pool and no-pool) route through it. Guarded to `intervention in {route, route2}`
+(only those have a quarantine); frac=0 collapses to a single plain generate, so
+vanilla/erase and all existing runs are byte-identical.
+
+**Verified.** `just smoke-route2 --rollout-ablate-frac=0.5`: 30 steps, clean exit,
+deploy eval fired (steps 0/10/20/29), all route2 columns populate, the ablated/full
+split padded+concatenated with no shape error.
+(log: `logs/20260601T053045_smoke_routing2_seed41.log`)
+
+**Next.** Queue route2-balanced + `--rollout-ablate-frac=0.5` (seed 41, 60 steps)
+and read `slv_dep`: the direct test of whether the exploration floor lifts deploy-solve
+vs the no-floor job 60. Keep the orthogonal `hkgap`-decay question (frozen vs
+`--vhack-refresh-every=2`) as a separate run so the two levers stay attributable.