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.

research-wiki/log.md

@@ -94,3 +94,6 @@
 - [2026-08-06 14:58 UTC] 新增 plan: issue #8 实施计划: stall 非生产性等待口径 (plan:issue8-stall-budget-plan)
 - [2026-08-06 14:58 UTC] 新增边: plan:issue8-stall-budget-plan --implements--> design:issue8-stall-budget
 - [2026-08-06 14:58 UTC] 重建索引: 61 篇页面
+- [2026-08-07 15:20 UTC] 新增 design: 建表前先探测,判死只认「确定写不进去」 (design:issue9-telemetry-ddl-probe)
+- [2026-08-07 15:20 UTC] 重建索引: 62 篇页面
+- [2026-08-07 15:11 UTC] 重建索引: 62 篇页面