Demote promoted processes when RSS drops below threshold

Previously, once a process was promoted to HIGH, it stayed HIGH forever
even if its RSS dropped well below the -mem threshold. Now promoted
processes are demoted back to NORMAL when RSS < -mem, and removed from
the promoted map.

Also: added TZ.md with full technical specification of the project.

main.go

@@ -538,10 +538,6 @@ func main() {
 				continue
 			}
 
-			if _, ok := promoted[p.PID]; ok {
-				continue
-			}
-
 			h, err := openProc(p.PID, ProcessQueryLimitedInformation)
 			if err != nil {
 				continue
@@ -556,6 +552,7 @@ func main() {
 							p.Name, p.PID, formatMemSize(rssBytes), prioName(origPrio))
 					} else {
 						if trySetPrio(h, p.PID, p.Name, PriorityClassNormal) {
+							delete(promoted, p.PID)
 							logf("DEMOTE %s (PID %d) RSS=%s, %s -> NORMAL",
 								p.Name, p.PID, formatMemSize(rssBytes), prioName(origPrio))
 						}