fix(deck): address PR #759 round-3 review feedback

- Drop "(in-place)" from filter-helper docstrings; callers should
  consume the return value, mutation is an implementation detail.
- Document that deck_get_archived_stacks always returns cards (an
  archived stack without its cards has no audit value); point to
  description_max_length for size control.
- Document that deck_get_cards applies filtering client-side, so it
  is network-equivalent to deck_get_stack(include_cards=True).
- Pin the empty-list contract: a stack with all-archived cards and
  include_archived_cards=False yields cards == [] (loaded but empty),
  not cards is None (explicitly suppressed).
- Add explicit one-character-over-limit truncation test alongside the
  existing exact-boundary test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_deck_server.py

@@ -122,6 +122,15 @@ def test_truncate_card_descriptions_at_exact_boundary():
     assert cards[0].description == "x" * 100
 
 
+def test_truncate_card_descriptions_one_over_limit():
+    """A description one character over the limit triggers truncation."""
+    cards = [_make_card(1, "x" * 101)]
+    _truncate_card_descriptions(cards, 100)
+    assert cards[0].description is not None
+    assert len(cards[0].description) == 101  # 100 chars + ellipsis
+    assert cards[0].description.endswith("…")
+
+
 def test_truncate_card_descriptions_shorter_than_limit_no_ellipsis():
     """A description shorter than the limit must not have an ellipsis appended."""
     cards = [_make_card(1, "hello")]
@@ -296,6 +305,28 @@ def test_apply_stack_filters_handles_none_cards():
     assert result.cards is None
 
 
+def test_apply_stack_filters_all_archived_yields_empty_list_not_none():
+    """A stack whose cards are all archived yields cards == [], not None.
+
+    Pin the contract: include_cards=True with all cards filtered out
+    means "the stack was loaded but had nothing to show", which is
+    semantically distinct from include_cards=False (cards=None,
+    "explicitly suppressed"). Callers checking ``stack.cards is None``
+    can use that to distinguish the two states.
+    """
+    stack = _make_stack(
+        cards=[_make_card(1, archived=True), _make_card(2, archived=True)]
+    )
+    result = _apply_stack_filters(
+        stack,
+        include_cards=True,
+        include_archived_cards=False,
+        description_max_length=None,
+    )
+    assert result.cards == []
+    assert result.cards is not None
+
+
 # _apply_card_filters -------------------------------------------------------