Chronology Protection as a Fundamental Law: From Double-Slit Erasure to History-Tagged Invariants — Comprehensive Rigorous Paper (v2 with Epilogue)

v2 adds Epilogue: Newton, Einstein, and the Next Discrepancy, and a new toy model in Chapter 3. We propose that Hawking's Chronology Protection Conjecture should be grounded in quantum mechanics rather than remain only a conjecture about general relativity. We interpret measurement in the double-slit experiment as a manifestation of a chronology protection law: the universe enforces non-contradiction. This paper makes the proposal mathematically precise where it can be made precise, and honest where it cannot. Proved in this paper: (i) an obstruction class [c] != 0 in H^2(G_tot, A) via bar resolution and cup product, and (ii) decidability of history-equality for regular histories in O(n log n) via Hopcroft minimisation. The protection law itself is formulated as a definition plus a testable suppression factor A_protected = A * exp[-lambda V S_loop], not as a derived theorem of standard quantum mechanics. It is a proposed principle + testable factor, not a proved derivation. Lambda has no measured value yet. New in v2, Chapter 3.3: 2-path interferometer toy model deriving the suppression from decoherence per loop, with table for N = 1, 5, 10, 50, 100. The remaining chapters give models and sketches with explicit gaps for H^2 identification, K0, Ext^2, tilting, classifying topos, and S-matrix correction. No full formalization in Lean of H^2, D^b, topos theory, or quantum field theory is claimed. Lean is used only for value-checks on the E,T pair. Status guide: Proved = complete proof given. Definition = stipulated formalism. Model = analogy, not proof. Sketch = outline with explicit gap. Conjecture = precise statement not proved here.