One-sided padding lemma: P112 to P120 at cost exactly +8 (L120 suffices for Delta >= n_nf)

[DreamingOfClouds]

Adds research/one-sided-padding-lemma/. No kappa claim.

• Padding: any one-sided P112 decoder of cost c becomes a one-sided P120 decoder of cost exactly c + 8 (merge corrected terms per i, then add (delta_{f_j}, y = 2e_j) on each frame line f_j with j not in I; q = C_E 2e_j is the E8 frame codeword).
• Lift form: cost_w(S+) <= cost_w(S) + 8 - |I|.
• Consequence (sufficiency only): L120 (every lift S containing C(E8) has cost_w >= 72) suffices for the one-sided P112 floor 64 + |I| for every I, i.e. Delta >= n_nf for all Delta, via the Decoder floors, case (b) for every number of terms: one-sided P112 decoders cost ≥ 64 without corrections (unique optimum) and with ≤ 2 non-full corrections (no κ claim) #2816 section 7 identity. The converse is not claimed. This makes Two-sided deficiency budget: Theorem BX for b_Q <= 4 (conditional frame, exact residue, no κ claim) #3029 conditional only on L120.
• Remark: delta delta^T padding is two-sided (delta is not a codeword; codewords have |Off| >= 53).
• OPEN: L120 itself. Residue: the 38 OPEN rows of the Research: one-sided residue closures, part 2 — Lemma Q4, count lemma + E6 dim-cover and rerunnable span passes close 78 more of the 116 open residues (38 open; no kappa claim) #3049 ledger, the 7 prior-only rows, the 7 E6-slack rows, and the d=0 sliver Z = ker rho cap S nonzero (kappa <= 6).
• Checks: bash verify.sh (needs sympy) runs two self-contained implementations (pad.py, pad_audit.py), 12+ decoders, exact over Q, one-sided, cost +8 by two independent cost computations. ALL PASS. The two scripts share one standard E8 root/label construction; the decoders and the two cost computations (direct F2 rank of B[R,A], and subspace algebra) are independent.

Cites #2816, #2826, #2862, #2985, #3049, #3029.

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