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, 49 insertions, 0 deletions

diff --git a/ep_run/holo_fast_probe.py b/ep_run/holo_fast_probe.py
new file mode 100644
index 0000000..9387199
--- /dev/null
+++ b/ep_run/holo_fast_probe.py
@@ -0,0 +1,49 @@
+"""Disambiguate the parity gap: real bug vs FD-amplified fp noise.
+(a) single-eval exactness: at a synthetic two-phase state Z=[zs+d, zs-d], compare the full doubled-batch
+    correction (Jv-JTv) against the halved+mirrored one. Exact math => allclose at fp level.
+(b) estimator noise floor: perturb zs by 1e-6 relative and rerun the ORIGINAL holo_a_track — if a_best
+    moves by ~the same 0.4 rel, the parity gap is the estimator's intrinsic FD sensitivity, not a bug."""
+import torch, torch.func as tf
+import lt_ep_train as L
+from holo_ep import holo_a_track
+
+torch.manual_seed(0)
+blk = L.EQBlock(512, 16, 256, 256, c=1.0, attn_mode='thick'); blk.qknorm = True
+ck = torch.load('runs/redx_traj/s2000.pt', map_location=L.dev)
+with torch.no_grad():
+    for p, w in zip(blk.allp, ck['allp']):
+        p.copy_(w.to(L.dev))
+torch.manual_seed(42)
+idx, y = L.get_batch('train', 24, 256)
+xin = blk.embed(idx).detach()
+zs = L.relax(blk, xin.clone(), xin, 150, 0.1)
+B = zs.size(0)
+fnc = lambda zz: blk.nc_force(zz)
+
+# (a) single-eval exactness
+torch.manual_seed(7)
+d = 0.02 * torch.randn_like(zs)
+Z = torch.cat([zs + d, zs - d], 0)
+zbar = 0.5 * (Z[:B] + Z[B:])
+zb2 = torch.cat([zbar, zbar], 0)
+v = (Z - zb2).contiguous()
+with torch.no_grad():
+    _, Jv = tf.jvp(fnc, (zb2,), (v,))
+    JTv = tf.vjp(fnc, zb2)[1](v)[0]
+    corr_full = Jv - JTv
+    v0 = (Z[:B] - zbar).contiguous()
+    _, Jv0 = tf.jvp(fnc, (zbar,), (v0,))
+    JTv0 = tf.vjp(fnc, zbar)[1](v0)[0]
+    corr_half = torch.cat([Jv0 - JTv0, -(Jv0 - JTv0)], 0)
+rel_a = ((corr_full - corr_half).norm() / (corr_full.norm() + 1e-12)).item()
+print(f"(a) single-eval: rel={rel_a:.2e}  ({'EXACT — gap is fp/FD noise' if rel_a < 1e-4 else 'REAL BUG'})", flush=True)
+
+# (b) estimator noise floor of the ORIGINAL
+r, T2, eps = 0.02, 40, 0.1
+a_ref, _ = holo_a_track(blk, zs, xin, y, r, T2, eps)
+zs_p = zs + 1e-6 * zs.norm() / (zs.numel() ** 0.5) * torch.randn_like(zs)
+a_prt, _ = holo_a_track(blk, zs_p, xin, y, r, T2, eps)
+rel_b = ((a_prt - a_ref).norm() / (a_ref.norm() + 1e-12)).item()
+cos_b = torch.nn.functional.cosine_similarity(a_ref.flatten(), a_prt.flatten(), dim=0).item()
+print(f"(b) orig-vs-orig under 1e-6 state noise: rel={rel_b:.2e} cos={cos_b:.6f}", flush=True)
+print("HOLO_FAST_PROBE_DONE", flush=True)