docs: unify language switchers - current language plain, other language linked (match README)
diagnostics pages: match the existing language-switch header format
land the diagnostics documentation (trace / CPU simulation / filing a report) Three new paired pages (zh primary + en): - Diagnostics: in-game commands, trace file locations, replay entry points, configuration; - Diagnostics-Simulation: verdict classification (COMPLETE/S1-S4), the two-file out-of-game CLI (mod jar + replay-shim jar), and the operation-by-operation semantics reference against the AE2UEL CraftingCPUCluster; - Diagnostics-Report: the evidence walkthrough (record next, reproduce, list, upload/download) with screenshot slots for live captures. Sidebar gains a diagnostics section.