docs: Paper B G2b cell + residual reframe

docs/paper.adoc

@@ -288,10 +288,11 @@ The Arabic client line has since been extended to a complete, confidence-bracket
 |layerdrop (enc 12→6, Muon, 6 ep) |190M |5.78 |[3.03, 3.49]
 |2.0 rung (Muon, 3 ep) |300M |4.82 |[2.36, 2.82]
 |2.1 rung (Muon, 6 ep) |300M |4.57 |[1.91, 2.35]
+|2.1 + full Tashkeela (G2b) |300M |4.82 |[2.19, 2.55]
 |teacher r7 |580M |2.29 |—
 |===
 
-Intervals are disjoint between adjacent rungs end to end: the frontier's separations are statistically real, not seed noise. Two structural findings sit on it. First, *pretrained width is load-bearing; depth is the compressible axis*. SVD width-stitching of the pretrained ByT5-small fails at both tested ratios (3.8× narrow: 82.96 — worse than the from-scratch collapse; 2× narrow: 78.23), while a verbatim layer copy of encoder layers 12→6 trains to 5.78 full-set — a shippable rung at 63% of the parameters whose 1.21pp depth cost at 6 epochs is CI-separated from the full-depth peer. Width surgery destroys the pretrained representation; depth surgery spends it. Second, the epochs lever is real but secondary: doubling 3→6 epochs moves the rung 4.82→4.57 (−0.25pp) — most of the residual is not undertraining. A pre-registered causal test of the remaining domain-coverage attribution swapped 8k news-domain units for classical-register Tashkeela at constant 30k budget and came back *negative* (5.81, −0.98pp vs control): register mix at constant budget does not fix the residual. The add-direction variant — 48k total with the full cleaned Tashkeela corpus (5× classical coverage, all other levers held) — is the live test of the domain-coverage attribution.
+Intervals are disjoint between adjacent rungs end to end: the frontier's separations are statistically real, not seed noise. Two structural findings sit on it. First, *pretrained width is load-bearing; depth is the compressible axis*. SVD width-stitching of the pretrained ByT5-small fails at both tested ratios (3.8× narrow: 82.96 — worse than the from-scratch collapse; 2× narrow: 78.23), while a verbatim layer copy of encoder layers 12→6 trains to 5.78 full-set — a shippable rung at 63% of the parameters whose 1.21pp depth cost at 6 epochs is CI-separated from the full-depth peer. Width surgery destroys the pretrained representation; depth surgery spends it. Second, the epochs lever is real but secondary: doubling 3→6 epochs moves the rung 4.82→4.57 (−0.25pp) — most of the residual is not undertraining. Two pre-registered causal tests closed the domain-coverage attribution. The swap direction — 8k news-domain units replaced by classical-register Tashkeela at constant 30k budget — came back *negative* (5.81, −0.98pp vs control). The add direction — 48k total with the full cleaned Tashkeela corpus (5× classical coverage, all other levers held, 39,018 steps) — came back *flat-negative*: 4.8231 full-set, delta vs teacher 2.3717 [2.194, 2.554], statistically indistinguishable from the 2.1 rung (4.5701 [1.91, 2.35]) with the point estimate 0.25pp worse. Both directions of the classical-corpus lever fail: the residual is *not* a classical-domain coverage deficit. It reframes as a teacher–student interaction the corpus cannot reach — candidate levers are on-policy distillation (in flight) and label-distribution effects, not more in-domain text.
 
 == The decode protocol is part of the measurement
 [[section-decode]]