fix: subset selection validation and remove unnecessary hash computation

Two bugs fixed in animal selection with checkboxes:

1. Confirmation dialog showed wrong count (e.g., "35 animals" instead of
   "2 animals" when only 2 were selected). Fixed by using valid_selected
   count in diff.server_count instead of full filter resolution count.

2. Spurious "Selection Changed" dialogs due to race condition in async
   hash computation. Fixed by removing client-side hash computation
   entirely - it was unnecessary since the server validates selected_ids
   directly against the filter resolution.

Changes:
- validation.py: Remove hash comparison in _validate_subset(), validate
  IDs directly, fix server_count in diff
- animal_select.py: Remove computeSelectionHash(), hidden roster_hash
  field, and related async fetch code
- test_selection_validation.py: Add tests for subset mode validation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_selection_validation.py

@@ -456,3 +456,86 @@ class TestSelectionMismatchError:
 
         assert error.result is result
         assert error.result.diff is diff
+
+
+# ============================================================================
+# Tests for validate_selection - subset mode
+# ============================================================================
+
+
+class TestValidateSelectionSubsetMode:
+    """Tests for validate_selection with subset_mode=True."""
+
+    def test_subset_mode_returns_valid_when_all_selected_match(
+        self, seeded_db, animal_service, strip1_location_id
+    ):
+        """validate_selection returns valid=True when all selected IDs are in filter."""
+        # Create cohort of 5 animals
+        ts_utc = int(time.time() * 1000)
+        payload = make_cohort_payload(strip1_location_id, count=5)
+        event = animal_service.create_cohort(payload, ts_utc, "test_user")
+        all_ids = event.entity_refs["animal_ids"]
+
+        # User selects only 2 of them
+        selected_ids = all_ids[:2]
+        subset_hash = compute_roster_hash(selected_ids, None)
+
+        ctx = SelectionContext(
+            filter="species:duck",
+            resolved_ids=all_ids,  # Full filter resolution
+            roster_hash=subset_hash,  # Hash of selected subset
+            ts_utc=ts_utc,
+            from_location_id=None,
+            subset_mode=True,
+            selected_ids=selected_ids,
+        )
+
+        result = validate_selection(seeded_db, ctx)
+
+        assert result.valid is True
+        assert result.resolved_ids == selected_ids
+        assert result.diff is None
+
+    def test_subset_mode_diff_server_count_is_valid_selected_count(
+        self, seeded_db, animal_service, strip1_location_id, strip2_location_id
+    ):
+        """In subset mode, diff.server_count should be count of valid selected IDs, not full filter."""
+        # Create cohort of 5 animals
+        ts_create = int(time.time() * 1000)
+        payload = make_cohort_payload(strip1_location_id, count=5)
+        event = animal_service.create_cohort(payload, ts_create, "test_user")
+        all_ids = event.entity_refs["animal_ids"]
+
+        # User selects 2 animals
+        selected_ids = all_ids[:2]
+        subset_hash = compute_roster_hash(selected_ids, None)
+
+        # Move one selected animal away (makes it invalid for the filter)
+        ts_move = ts_create + 1000
+        move_payload = AnimalMovedPayload(
+            resolved_ids=[selected_ids[0]],
+            from_location_id=strip1_location_id,
+            to_location_id=strip2_location_id,
+        )
+        animal_service.move_animals(move_payload, ts_move, "test_user")
+
+        # Now validate at ts_move - one of the selected animals is no longer at Strip 1
+        ctx = SelectionContext(
+            filter="location:'Strip 1'",
+            resolved_ids=all_ids,  # Full filter resolution at creation time
+            roster_hash=subset_hash,
+            ts_utc=ts_move,  # Validate at move time
+            from_location_id=None,
+            subset_mode=True,
+            selected_ids=selected_ids,
+        )
+
+        result = validate_selection(seeded_db, ctx)
+
+        assert result.valid is False
+        assert result.diff is not None
+        # BUG: diff.server_count is currently len(resolved_ids) = 4 (5 minus moved)
+        # SHOULD BE: len(valid_selected) = 1 (2 selected minus 1 moved)
+        assert result.diff.server_count == 1  # Only 1 valid selected animal remains
+        assert result.diff.client_count == 2  # User selected 2
+        assert selected_ids[0] in result.diff.removed  # The moved animal is invalid