holofast: exact halved-jvp AEP track — 1.55x nudged phase, gradient-gate passed

holo_a_track computed the doubled-batch jvp/vjp on [v0; -v0] at a shared
anchor zbar — exact antisymmetric redundancy (phase deviations from the
common mode are exact negatives). holo_a_track_fast computes at batch B and
mirrors: single-eval parity 6e-7 (exact); trajectory-level 45% divergence
SHARED with the original's own FD noise floor (1e-6 state noise -> 49%
self-divergence — the 2r=0.04 finite difference amplifies fp noise; training
averages it via pema/momentum). Ship gate: cos(EP,BPTT) orig vs fast
indistinguishable (0.907/0.912, 0.853/0.853, 0.918/0.918). Timing 6.43->4.16s
on the T2=40 nudged phase (contended GPU, relative). --holofast flag,
default off; queued ablation arms deliberately stay on orig for fidelity.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014FAPDWQ49M5Ye3NpTndTpn

1 files changed, 5 insertions, 2 deletions

diff --git a/ep_run/lt_ep_train.py b/ep_run/lt_ep_train.py
index d56af1d..e7155d3 100644
--- a/ep_run/lt_ep_train.py
+++ b/ep_run/lt_ep_train.py
@@ -180,12 +180,13 @@ def ep_step(blk, idx, y, T1, T2, eps, beta, jacreg=0.0, holo=0, hr=0.02, t1max=0
                 z = z + eps * f
         return z.detach()
     if holo == 2 and t2sel > 0:                           # adaptive-T2, phase-batched fast path (validated ==)
-        from holo_ep import holo_a_select2, holo_a_track
+        from holo_ep import holo_a_select2, holo_a_track, holo_a_track_fast
         K = max(1, getattr(blk, 'navg', 1))               # restart-averaging: noise / sqrt(K)
         acc = None
         for _ in range(K):
             if getattr(blk, 'track', False):              # common-mode-tracking AEP (loose-tolerant)
-                ai, _ = holo_a_track(blk, zs, xin0, y, hr, t2sel, eps)
+                _tr = holo_a_track_fast if getattr(blk, 'holofast', False) else holo_a_track
+                ai, _ = _tr(blk, zs, xin0, y, hr, t2sel, eps)   # fast = exact halved-jvp mirror (1.55x)
             else:
                 ai, _ = holo_a_select2(blk, zs, xin0, y, hr, t2sel, eps, li=getattr(blk, 'li_avg', 0))
             acc = ai if acc is None else acc + ai
@@ -419,6 +420,7 @@ def main():
     ap.add_argument('--li_avg', type=int, default=0)           # lock-in integration window (0=snapshot mode)
     ap.add_argument('--navg', type=int, default=1)             # restart-averaged contrast estimates per update
     ap.add_argument('--track', action='store_true')            # common-mode-tracking AEP correction
+    ap.add_argument('--holofast', action='store_true')         # exact halved-jvp track (1.55x nudged phase; parity = FD noise floor)
     ap.add_argument('--rt_final', type=float, default=0.0)     # anneal res_target to this (0=off), 25%-75% of run
     ap.add_argument('--nudge_brake', type=float, default=0.0)  # kappa: anchor spring during nudge (Tikhonov adjoint)
     ap.add_argument('--init_ckpt', type=str, default='')       # warm-start weights from a saved ckpt
@@ -493,6 +495,7 @@ def main():
     blk.li_avg = cfg.li_avg
     blk.navg = cfg.navg
     blk.track = cfg.track
+    blk.holofast = cfg.holofast
     blk.nbrake = cfg.nudge_brake
     blk.qknorm = cfg.qknorm
     if cfg.resinit != 1.0:                          # near-identity block at init (contractive) -> stable big-width start