Paper CQ-IV: Horizon Thermodynamics of the Sector-I/IV Boundary, Topological Information Protection, and the FSC Page Curve

We develop the thermodynamic and quantum-information structure of the Sector-I/IV boundary in the Four-Sector Cosmological (FSC) framework, building on the complex-time geometry of Papers A-E and the [[4,1,2]] stabiliser-code interpretation of CQ-III. The boundary at theta=-pi/4 is identified as a Killing horizon of the FSC static metric with surface gravity kappaIV=lambda and Hawking temperature T_H^FSC = hbar*lambda/2pi*k_B ~ 1.52e-29 K. Together with the Planck-scale result of Paper TS-I, this establishes a two-tier thermodynamic hierarchy spanning 61 orders of magnitude. Using the complete entropy table S(R)=n_WH(R)*log 2 of CQ-III, all erasure channels are classified into Pathway A (generic, S=2log 2) and Pathway B (wormhole-axis, S=0). Pathway B provides topological protection of quantum information via the wormhole structure, identified as the FSC analogue of ER=EPR and the Island formula. The FSC Page time t_Page^FSC = (ln 2)/lambda ~ 1.76 Gyr is derived without free parameters. Under a transfer-map hypothesis, the flat Schmidt spectrum (lambda_1=lambda_2=1/2) of the [[4,1,2]] code predicts C_ellEE/C_ellBB->1 in the primordial-tensor regime ell<=100 (Prediction P6, testable by LiteBIRD and CMB-S4). Five open questions (OQ-CQ4-1 through OQ-CQ4-5) are delineated for Paper CQ-V.