Correction 3: gate and guard (low)

The gate now re-hashes every pinned provenance file (all 33, and it rejects any unpinned provenance file), every pinned code/conventions file and the bound independent report at gate time. It also checks that every file it reads is in the source set or is a build output. The d guard now refuses value assignment (evaluate/subs/__call__/compose) on PolyRing/FracField elements whose generators include $d$ when called directly. All 8 reviewer forms raise, and the 18 earlier forms still raise. Disclosed residual: raw dense-list evaluation carries no symbol information and is not guarded; no package code uses it.