vocab: stop re-teaching a word later in the same session (239 tags)

A word tagged VOCAB in one activity was tagged again in later activities of the
same session, so the learner got the same chip repeatedly — most visibly in the
pronunciation drills, which re-listed every word from the dialogue before them
(reported: book 7 session 1-2). Rule applied: drop a VOCAB whose word was
already tagged in an EARLIER activity of that session. Repeats INSIDE one
activity are kept — those teach distinct senses ('stuck to his finger' vs
'stuck the milk in the fridge', 'shop' as verb then noun). Also dropped chips
for plurals of already-known words in book 7 session 1-2, where the drill
teaches the singular/plural pair rather than new vocabulary. No session lost
all of its vocabulary (verified).

Co-authored-by: Cursor <cursoragent@cursor.com>

book-7-lesson-4.module

@@ -850,8 +850,6 @@ ID: l4-s9
 INTRO: Jane and Mark's carpet is old and ugly, and Jane has decided its time is up. Read their conversation.
 INSTRUCTION: Read the dialog.
 
-VOCAB: ugly
-VOCAB: pretty
 Jane: This carpet is old and ugly. We need a new, pretty one.
 
 VOCAB: to lay | lay