End the hint prompt in a full stop: an unterminated list made Whisper drop punctuation and loop

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/hint.ts

@@ -72,10 +72,13 @@ function defaultStorage(): StorageLike | null {
  * Token ids for the hint as a Whisper prompt (without `<|startofprev|>`),
  * truncated predictably to `budget` tokens (D-WS2a):
  *
- * 1. Encode `" " + normalizeHint(hint)` (Whisper prompts start with a space).
+ * 1. Prompt text = `" " + normalizeHint(hint)` (Whisper prompts start with a
+ *    space), ending in a full stop unless it already ends in . ? or ! —
+ *    measured: an unterminated list makes Whisper drop punctuation and, on
+ *    smaller models, fall into repetition loops (D-WS2a).
  * 2. Over budget → keep the longest prefix of whole space-separated words that
- *    fits, with a dangling "," / ";" removed: the start of the list is kept,
- *    the end dropped, never mid-word.
+ *    fits (full stop included), a dangling "," / ";" removed: the start of the
+ *    list is kept, the end dropped, never mid-word.
  * 3. Not even the first word fits → its first `budget` tokens.
  *
  * `encode` must add no special tokens. Mirrored by `fit_prompt_ids` in
@@ -88,12 +91,12 @@ export function fitHintToBudget(
 ): number[] {
   const text = normalizeHint(hint);
   if (text.length === 0) return [];
-  const full = encode(` ${text}`);
+  const full = encode(promptSentence(text));
   if (full.length <= budget) return full;
 
   const words = text.split(" ");
   const encodePrefix = (count: number) =>
-    encode(` ${words.slice(0, count).join(" ").replace(/[,;]+$/, "")}`);
+    encode(promptSentence(words.slice(0, count).join(" ").replace(/[,;]+$/, "")));
   // Largest word count whose encoding fits; token count grows with the
   // prefix, so binary search needs O(log n) encodes even for huge pastes.
   let low = 0;
@@ -106,3 +109,7 @@ export function fitHintToBudget(
   if (low === 0) return encode(` ${words[0]}`).slice(0, budget);
   return encodePrefix(low);
 }
+
+function promptSentence(text: string): string {
+  return /[.?!]$/.test(text) ? ` ${text}` : ` ${text}.`;
+}