Fixes #33 — verb examples must actually use the verb

The example sentences in VerbDetailView (and the new vocab practice
modes) frequently used the wrong verb — a "tener" set would show
"Él estaba leyendo un libro" (estar), "Nosotros vamos a viajar" (ir),
"Tú debes estudiar" (deber). The model drifted off the target verb
partway through generating the 6-example batch, and nothing checked
the output.

Two defenses:

  Prompt grounding — VerbExampleGenerator.generate now takes a
  formsByTense map (tenseId → conjugated forms, from the new
  ReferenceStore.conjugatedForms). Each tense line in the prompt
  lists the verb's exact conjugated forms and instructs the model to
  use one of them. The model echoes a real form instead of recalling
  (and mis-recalling) the conjugation.

  Output validation — every generated sentence is checked against the
  conjugation table via accent/case-folded whole-word matching. Any
  sentence that doesn't contain a real conjugated form of the verb is
  rejected. Failures trigger one regeneration pass; anything still
  wrong is dropped rather than displayed. Better to show 4 correct
  examples than 6 with 2 wrong.

  Cache invalidation — VerbExampleCache now persists a versioned
  wrapper (version 2). Pre-fix cached example sets — which may contain
  wrong-verb sentences — fail the version check and are discarded, so
  they regenerate cleanly under the new path.

Callers updated: VerbDetailView, VocabFlashcardPracticeView,
VocabMultipleChoicePracticeView all build formsByTense from
ReferenceStore and pass it through.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Services/ReferenceStore.swift

@@ -94,6 +94,19 @@ struct ReferenceStore {
         return (try? context.fetch(descriptor)) ?? []
     }
 
+    /// Map of tenseId → conjugated forms for a verb, used to ground and
+    /// validate LLM-generated example sentences.
+    func conjugatedForms(verbId: Int, tenseIds: [String]) -> [String: [String]] {
+        var map: [String: [String]] = [:]
+        for tenseId in tenseIds {
+            let forms = fetchForms(verbId: verbId, tenseId: tenseId)
+                .map(\.form)
+                .filter { !$0.trimmingCharacters(in: .whitespaces).isEmpty }
+            if !forms.isEmpty { map[tenseId] = forms }
+        }
+        return map
+    }
+
     func fetchForm(verbId: Int, tenseId: String, personIndex: Int) -> VerbForm? {
         let descriptor = FetchDescriptor<VerbForm>(
             predicate: #Predicate<VerbForm> { form in