phraser, mavend: read the fallbacks from the file (V-501)

The accessors are functions now, so the call sites that compared against one
literal compare against the entry instead: IsUnknownFallback and
IsSourcesFallback in the daemon tests, the entry key in the phraser tests. A
reworded variant no longer breaks a Go test.

The eval scores every variant on the persona checks the nudges already pass.

cmd/mavend/worldmodel.go

@@ -23,7 +23,11 @@ type worldPhraser interface {
 // question about his meeting came back as a swimming competition in Nottingham.
 // Naming the gap is the rule CLAUDE.md already applies to a sibling service
 // being down.
-const worldGap = "сейчас не могу ответить — большая модель недоступна, а придумывать не хочу."
+//
+// The wording lives in fallbacks_ru_v1.json and is fixed there, not picked from
+// variants: this sentence names one specific gap and must not drift into a
+// general "I don't know".
+func worldGap() string { return phraser.WorldGap() }
 
 // phraseWorld asks the world model, or reports the gap.
 //