frame gate: empty-period rotations and spectra half-turns are not frame mixing

[bvkay]

Two export shapes were refused as per-period frame mixing and are not. EMpower writes a rotation of 0 at periods whose tipper or impedance is the EMPTY sentinel and the real angle (8 or 41 degrees) on every estimate; the gate now reads rotation angles only at periods that carry data. Bollards Lagoon BL02 and BL11 carry ROTSPEC=180 with HX written 360; negating both horizontal axes leaves every cross-power and so the impedance unchanged, and both stations agree with their 2 km neighbours to 2 to 4 degrees of phase, so a half-turn serves in the azimuth frame with the tipper sign noted. Local rebuild of the four affected packages: 260 stations built, only Renmark RM0111 and RM011B still refused (a real axis swap). Four new gate tests; policy header and build-lifecycle docs updated.