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-6-lesson-4.module

@@ -170,7 +170,6 @@ ID: l4-s3
 INTRO: The Whites need somewhere to sleep. Mr. White calls the Hotel Royale to make a reservation — listen for "check in" and "check out".
 INSTRUCTION: Listen to the dialog.
 
-VOCAB: hotel
 Hotel_Clerk: Hotel Royale, may I help you?
 Mr_White: This is Ben White. I'd like to make a reservation. My wife and I need a room. We'd like to stay at the hotel from the third to the eighth of August.
 
@@ -199,7 +198,6 @@ Mrs_White: Because our vacation begins tomorrow. We can relax at last. This is f
 Mr_White: What is it?
 Mrs_White: Open it.
 
-VOCAB: camera
 VOCAB: rolls of film
 Mr_White: A camera and five rolls of film!