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.

src/polygateway/ports.py

@@ -284,7 +284,7 @@ class TelemetryStatusProvider(Protocol):
 
 @runtime_checkable
 class TelemetryRecorder(Protocol):
-    """遥测后端;36 字段冻结(M1 设计 §4.4 + issue #3/#4/#11/#16/#20 + 1.3.5),唯一调用点是 TelemetryEmitter。
+    """遥测后端;38 字段冻结(M1 设计 §4.4 + issue #3/#4/#11/#16/#20 + 1.3.5/1.3.8),唯一调用点是 TelemetryEmitter。
 
     新增参数不设默认值: 库外无第三方实现者(三项目迁移时删除了各自的同名
     Protocol),完整签名的成本为零,而少写一列会被 emitter 的降级吞成 warning。
@@ -349,4 +349,6 @@ class TelemetryRecorder(Protocol):
         error_body: str | None,
         attempts: int | None,
         total_latency_ms: int | None,
+        stall_ms: int | None,
+        stall_count: int | None,
     ) -> None: ...