Add animal_id filter support for registry selection

The registry selection feature builds filters like animal_id:X|Y|Z
but the parser didn't recognize animal_id as a valid field.

- Add animal_id to VALID_FIELDS in parser.py
- Add animal_id handler in resolver.py _build_filter_clause

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

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

src/animaltrack/selection/parser.py

@@ -7,7 +7,7 @@ from animaltrack.selection.ast import FieldFilter, FilterAST
 
 # Supported filter fields
 VALID_FIELDS = frozenset(
-    {"location", "species", "sex", "life_stage", "identified", "tag", "status"}
+    {"animal_id", "location", "species", "sex", "life_stage", "identified", "tag", "status"}
 )
 
 # Fields that can be used as flags (without :value)

src/animaltrack/selection/resolver.py

@@ -148,7 +148,16 @@ def _build_filter_clause(field_filter: FieldFilter, ts_utc: int) -> tuple[str, l
     field = field_filter.field
     values = list(field_filter.values)
 
-    if field == "species":
+    if field == "animal_id":
+        # Direct animal ID filter
+        placeholders = ",".join("?" * len(values))
+        query = f"""
+            SELECT animal_id FROM animal_registry
+            WHERE animal_id IN ({placeholders})
+        """
+        return query, values
+
+    elif field == "species":
         # Species from animal_registry (current state)
         placeholders = ",".join("?" * len(values))
         query = f"""