fix: probe for the telemetry table before creating it

PostgreSQL checks the schema CREATE privilege before the IF NOT EXISTS
existence test, so an account with only table-level INSERT was denied on
CREATE TABLE IF NOT EXISTS even though the table was right there and
writable. The denial set _failed and the whole recorder went no-op for
the process lifetime, silently: 150+ calls downstream lost their latency,
token and cost rows with nothing but one warning to show for it.

The probe is the direct fix. The larger fix is the criterion: structural
degradation now means "provably cannot write" (pool creation failed, or
the table is absent and cannot be created), not "something threw during
init" -- a probe or acquire failure just skips the row and retries on the
next call.

SQLite stays as it is on purpose. Measured: it short-circuits the
statement at parse time, so it passes even under another connection's
EXCLUSIVE lock or on a read-only file. A probe there would buy nothing;
the docstring now says so to keep symmetry-minded future edits away.

src/polygateway/telemetry/sqlite.py

@@ -3,6 +3,14 @@
 蓝本 VT `adapters/telemetry.py`: 构造期建连接与表,失败降级为 no-op
 (记录基础设施不得拖垮业务调用);`INSERT OR IGNORE` 幂等(call_id 主键);
 写入经 threading.Lock 串行化后由 `asyncio.to_thread` 执行,不阻塞事件循环。
+
+**这里不做 postgres.py 那样的建表前探测,是实测后的有意不对称**(issue #9):
+SQLite 对已存在的表在**解析期**就把 `CREATE TABLE IF NOT EXISTS` 短路掉,
+既不抢写锁也不检查可写性——实测同一时刻另一连接持 `BEGIN EXCLUSIVE`、或
+文件 `chmod 444`,该语句均通过,而同条件下的 `INSERT` 与新表名建表分别报
+database is locked / readonly database。故 PG 侧"权限检查早于存在性判断"
+的坑在此不存在,加探测零收益。**别为了代码对称把它加回来**;需要对称的是
+保证(表存在就不该因建表失败而失能),这一条两侧都已满足。
 """
 
 from __future__ import annotations