Let a lesson list review keys beside the two it introduces

A lesson's `keys` was capped at two outright, which made it impossible to write
"K, with the four keys before it still in play". The cap now counts only what is
*new*; keys already taught can be listed freely, and the spotlight falls on the
new ones alone, so a lesson titled "K" drills K instead of spreading itself
evenly over all five keys it names. A round that introduces nothing new still
spotlights its whole list - that is the "mixed" replay, unchanged.

Also documents `unlocks:` paths as relative to the library root. Absolute and
"~" paths still resolve, so an existing file keeps working, but a relative path
survives moving the library.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

python-backend/tippen-curriculum.yml.example

@@ -1,11 +1,14 @@
 # The typing game's lesson plan, referenced from config.yml's general.tippen.curriculum_file.
 #
 # Every lesson: title, subtitle (read aloud). Then either:
-#   - keys: the letters-kind key(s) this round drills. The loader tracks which keys were
-#     already active: the first time a key appears its lesson is "isolated" (heavy
-#     weight, alone); the second (identical) appearance is "mixed" (lighter weight,
-#     blended with everything learned so far). Two lessons with the same `keys` back to
-#     back is exactly how you write "isolated, then mixed" - no separate flag needed.
+#   - keys: the letters-kind keys this round has on the table. At most two of them may
+#     be new; the rest are keys already taught, written out to say "these are in play
+#     too". The loader tracks which keys were already active and spotlights only what is
+#     new, so `keys: [f, j, a, d, k]` right after `[f, j, a, d]` is a K lesson with the
+#     other four as review ("isolated" - heavy weight on K). A round that introduces
+#     nothing new spotlights its whole list instead ("mixed" - lighter weight, blended
+#     with everything learned so far), so two lessons with the same `keys` back to back
+#     is exactly how you write "isolated, then mixed" - no separate flag needed.
 #   - drill: true - a pure review round, no new content, whatever is active so far.
 #   - kind + words - a fragments/words/sentences consolidation round (dive mode by
 #     default; `mode:` overrides it - see the backend's `ELIGIBLE_MODES` for which
@@ -18,8 +21,11 @@
 #
 #   unlocks: <path>
 #
-# A path into the music library (see config.yml's general.library.root - absolute, or
-# relative to it; "~" is expanded). Passing this lesson (two stars, or five attempts
+# A path into the music library, written relative to config.yml's
+# general.library.root - "Musik/Klaviersammlung/Lied.mp3", not a path from "/" or "~".
+# (An absolute path, or one starting with "~", still resolves, so an old file keeps
+# working - but relative is the form to write, since it survives moving the library.)
+# Passing this lesson (two stars, or five attempts
 # regardless of score) unlocks that track and everything before it in the same album,
 # inclusive - so pointing at an album's last track unlocks the whole album, and
 # pointing at track 5 of an audiobook unlocks chapters 1 through 5. The web front-end
@@ -59,7 +65,7 @@ worlds:
         subtitle: "Die Ringfinger"
         keys: [s, l]
         # Example - unlocks tracks 1 through 5 (inclusive) of this album:
-        # unlocks: ~/Music/Musik/Kinderparty - Kinderparty Lieder/05 - Lied.mp3
+        # unlocks: "Musik/Kinderparty - Kinderparty Lieder/05 - Lied.mp3"
       - title: "S und L üben"
         subtitle: "Die neuen Tasten festigen"
         keys: [s, l]
@@ -195,7 +201,7 @@ worlds:
         drill: true
         # Example - unlocks a whole audiobook (its last chapter, inclusive of every
         # chapter before it):
-        # unlocks: ~/Music/Hörbücher/Conni - Conni in den Bergen/06 - Teil.mp3
+        # unlocks: "Hörbücher/Conni - Conni in den Bergen/06 - Teil.mp3"
 
   - number: 3
     title: "Nach unten"
@@ -295,7 +301,7 @@ worlds:
         drill: true
         # Example - unlocks a whole podcast show (its most recent episode, and every
         # earlier one of the same show, chronologically):
-        # unlocks: ~/Music/Kinderpodcasts/Wissen macht Ah/20260101 - Neu.mp3
+        # unlocks: "Kinderpodcasts/Wissen macht Ah/20260101 - Neu.mp3"
 
   - number: 4
     title: "Große Buchstaben"