feat(update): cache fetched release notes and strict-match patch notes version

tests/engine/launcher_patch_notes.lua

@@ -68,6 +68,12 @@ do
     "stashed notes name a release version")
 end
 
+do
+  local notes, ver = PatchNotes.fromRepo("999.999.999")
+  eq(notes, nil, "fromRepo returns nil when a specific engine version is missing")
+  eq(ver, nil, "fromRepo version is nil when missing")
+end
+
 do
   local f = assert(io.open("mobile/ios/app-repo.json", "rb"))
   local list = PatchNotes.parseRepo(f:read("*a"))
@@ -78,6 +84,15 @@ do
   eq(notes, list[2].notes, "fromRepo returns that version's notes")
 end
 
+do
+  local oldVersion = package.loaded["src.core.Version"]
+  package.loaded["src.core.Version"] = { engine = "999.999.999" }
+  local body, ver = PatchNotes.body(nil)
+  eq(body, "Unable to fetch patch notes.", "returns Unable to fetch patch notes when version is uncached and unlisted")
+  eq(ver, "999.999.999", "returns the requested engine version")
+  package.loaded["src.core.Version"] = oldVersion
+end
+
 imp._appPatchNotes = true
 local modal = drawAndCapture(imp)
 check(modal:find("Patch notes", 1, true) ~= nil, "the modal titles itself")