Generic two-component / phosphorelay signal transduction motif

The minimal, reusable phosphorelay motif of two-component signal transduction: a sensor histidine kinase autophosphorylates on a conserved histidine in response to a stimulus, then transfers the phosphoryl group to a conserved aspartate on a cognate response regulator, which effects the output (often transcriptional). An optional histidine-phosphotransfer (Hpt) step inserts a His->Asp->His->Asp multistep relay between the sensor and the terminal response regulator. The motif is gene-free and taxon-neutral: it fixes the sensor-kinase / (phosphotransfer) / response-regulator roles by molecular-function term and the His->Asp phosphotransfer topology. Bacterial, archaeal, plant, and fungal two-component systems embed it through `conforms_to` (e.g. the Sln1->Ypd1->Ssk1 phosphorelay upstream of MODULE:scer_hog1_cascade). Grounded in GO:0000160 (phosphorelay signal transduction system).

Reusable conformance target. The canonical motif is the linear multistep phosphorelay sensor -> Hpt -> response regulator. A multistep phosphorelay (e.g. the yeast Sln1 -> Ypd1 -> Ssk1 system) conforms EXACT. A simple bacterial two-component system, which lacks a discrete Hpt domain and transfers directly from the sensor to the response regulator, declares `conforms_to: [{template: two_component_relay, status: WITH_DEVIATIONS}]` with a deviation noting the collapsed Hpt step. Participants are abstract selectors because the motif asserts the phosphorelay chemistry only.