The Weight That Could Not Break It: Machine-Checked Endpoint Reflection Positivity for the Coupled Z_2 Slice

WHAT IS PROVED, STATED BEFORE ANYTHING ELSE. The reflected two-point form ofthe Gibbs measure, at the two ENDS of a path, for REAL observables of a SINGLEslice, is non-negative under the parity and coupling hypotheses stated below-- at even separation for every beta, and at every separation exactly for beta>= 0 -- and the Gram matrix of a finite family of such observables is positivesemidefinite under the same hypotheses. THIS IS NOT YET THEOSTERWALDER-SCHRADER AXIOM, which quantifies over observables of the wholepast half-chain and over complex ones. The half-chain algebra, the reflectionmap and the sesquilinear form are not built here; the scope section says whatis missing and why it is a construction rather than a further inequality.Eleven papers in this lane end with REFLECTION POSITIVITY IS UNTOUCHED. Thistouches the endpoint form of it, and the interesting part is what the spatialsource weight does -- namely nothing. Earlier papers do prove statements aboutthe coupled kernel; specGap < lambda holds there too. What is new is that thisresult is UNCHANGED by the weight: same hypothesis, same conclusion, and noconstant that depends on w. Every earlier coupled-kernel statement contains anumber that moves when w does. Positivity is the SIGN of a quadratic form, andconjugation by sqrt(w) is a congruence, so there is nothing for the weight tomove.TWO REFLECTIONS, TWO HYPOTHESES. The reflected two-point sum is with v the dressed observable, and the two cases have genuinely differentcontent. Through a SITE (N even) the sum is a square, hence non-negative forEVERY beta -- negative coupling included -- with nothing used but symmetry ofthe kernel. Through a BOND (N odd) it is , which needs Kitself positive semidefinite; that holds exactly for beta >= 0, by aninduction on the extent.AND THE SECOND HYPOTHESIS IS ACTIVE. At beta < 0 and odd separation thesingle-site sign observable gives the reflected sum in closed form, 2(e^beta -e^-beta)^N, which is negative. Congruence is invertible, so the same witnessdivided by sqrt(w) shows the COUPLED kernel is indefinite below zero for EVERYstrictly positive weight, at every extent with at least one site. At L = 0there is nothing to witness -- one configuration, and there the decoupledkernel is 1 while the coupled one is the positive scalar w(empty) -- and thatexception is recorded rather than left to be found. The boundary beta = 0 issharp and exhibited, not inferred.WHAT THIS IS NOT. Neither the full axiom (above) nor any reconstruction: thephysical Hilbert space as the quotient of the past algebra by the null spaceof this form is not built. And the degeneration of specRatio(L) under a ringweight is MEASURED, not proved; no theorem here or elsewhere in the lane saysthe weight destroys uniformity, only that no uniform bound is proved for it.Nothing here concerns uniformity in the extent, SU(N), the continuum limit, orthe Yang-Mills mass gap.ON THE PRE-REGISTRATION. Three ACTIVE gates were committed before any Lean waswritten, and the gates section reports what happened to each. A fourth ispreserved there and not counted: the original Gate B, which failed because ofa design error of ours, and which two of the three active gates replaced.