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/integration/test_postgres_telemetry.py

@@ -69,6 +69,9 @@ _EXPECTED_COLUMNS = [
     "error_body",
     "attempts",
     "total_latency_ms",
+    # —— 1.3.8 沉默归因两列(issue #27)——
+    "stall_ms",
+    "stall_count",
 ]
 
 # 1.3.5 之前那张表的 27 个物理列(26 个 INSERT 字段 + created_at)。写成固定切片
@@ -769,7 +772,7 @@ class TestCallerDimensionsAcceptance:
                 "WHERE table_schema = $1 AND table_name = 'llm_calls' ORDER BY ordinal_position",
                 schema,
             )
-            # 22 → 36 个 recorder 字段(加 created_at 共 37 个物理列),且新列追加在末尾
+            # 22 → 38 个 recorder 字段(加 created_at 共 39 个物理列),且新列追加在末尾
             assert [r["column_name"] for r in cols] == _EXPECTED_COLUMNS
             rows = await _fetch(
                 schema_dsn,
@@ -1142,7 +1145,7 @@ class TestCallObservabilityColumnsAcceptance:
                 "WHERE table_schema = $1 AND table_name = 'llm_calls' ORDER BY ordinal_position",
                 schema,
             )
-            # 26 → 36 个 recorder 字段(加 created_at 共 37 个物理列),新列追加在末尾:
+            # 26 → 38 个 recorder 字段(加 created_at 共 39 个物理列),新列追加在末尾:
             # 列序与新建库一致才不会让两条升级路径分叉
             assert [r["column_name"] for r in cols] == _EXPECTED_COLUMNS
 
@@ -1337,7 +1340,7 @@ class TestPublishedSchemaScript:
 
         await _execute_script(fresh_dsn, script)
         actual = [r["column_name"] for r in await _fetch(fresh_dsn, _PHYSICAL_COLUMNS_SQL, schema)]
-        # 物理列 = 36 个 INSERT 字段 + 库从不显式写的 created_at;对着库常量比,不另抄一份
+        # 物理列 = 38 个 INSERT 字段 + 库从不显式写的 created_at;对着库常量比,不另抄一份
         assert set(actual) == set(COLUMNS) | {"created_at"}
         # 列序也不许漂: 新列必须排在 created_at 之后,否则新建库与 ALTER 升级的列序分叉
         assert actual == _EXPECTED_COLUMNS