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/tests/test_tippen_curriculum.py

@@ -117,19 +117,21 @@ def test_wrong_world_count_is_one_problem(tmp_path: Path) -> None:
 
 def test_multiple_problems_are_all_reported_at_once(tmp_path: Path) -> None:
     worlds = [dict(w) for w in VALID["worlds"]]
-    # Two independent problems in one file: a reused reward, and a lesson with too many
-    # keys - both should show up in a single error, not just the first one found.
+    # Two independent problems in one file: a reused reward, and a lesson introducing
+    # too many keys at once - both should show up in a single error, not just the first
+    # one found. ("a" and "s" are already taught in world 1, so only the three unseen
+    # keys count against the cap.)
     worlds[1] = {**worlds[1], "reward": "clownfish"}
     worlds[2] = {
         **worlds[2],
-        "lessons": [{"title": "Zu viel", "subtitle": "x", "keys": ["a", "s", "d"]}],
+        "lessons": [{"title": "Zu viel", "subtitle": "x", "keys": ["a", "s", "q", "w", "e"]}],
     }
     with pytest.raises(CurriculumError) as excinfo:
         load_curriculum(_write(tmp_path, {"worlds": worlds}))
     message = str(excinfo.value)
     assert "2 problems" in message
     assert "reuses reward" in message
-    assert "at most two keys" in message
+    assert "at most two new keys" in message
 
 
 def test_letters_kind_cannot_have_words(tmp_path: Path) -> None: