feat: add stall_ms and stall_count telemetry columns

Third approved port change: TelemetryRecorder.record_llm_call gains
stall_ms/stall_count (assembly gate derives from this signature).
Schema five edit points (both DDLs, both backfill decls, COLUMNS)
with INTEGER nullable columns appended last; telemetry_schema_sql
regenerates from the same constants untouched. Emitter forwards the
pair from retry._emit on the success branch only (cancelled hedge
loser rows stay NULL by construction); cache-hit and terminal rows
pass explicit None. Both **fields recorders pick the columns up with
zero signature change.

tests/unit/test_ports.py

@@ -147,6 +147,8 @@ class _DummyRecorder:
         error_body,
         attempts,
         total_latency_ms,
+        stall_ms,
+        stall_count,
     ) -> None: ...
 
 
@@ -319,7 +321,7 @@ class TestTelemetryRecorderSignature:
             "attempts",
             "total_latency_ms",
         } <= set(params)
-        assert len(params) - 1 == 36  # 减掉 self
+        assert len(params) - 1 == 38  # 减掉 self
 
     @pytest.mark.parametrize(
         "name",