fix: close() nulls _pool + connect() lock (psql-7/8 twin), % in identifiers, doc caveats

close() now nulls self._pool so a closed instance reports not-connected instead of
masquerading as live (twin of psql-7); connect() is guarded by an internal asyncio.Lock
so concurrent connect() calls serialize instead of racing to create and orphan multiple
live pools (twin of psql-8). _quote_ident now escapes a literal % in identifiers (mysql-7:
PyMySQL's query % args substitution otherwise breaks any Layer-1 call against a %-bearing
table/column name). fetchval/exists tolerate a caller-overridden tuple cursorclass via a
new _first_value helper (mysql-8). Documents the %%-escaping rule for literal % in raw SQL
text (mysql-6) and the upsert() VALUES() deprecation on MySQL 8.0.20+ (mysql-9).

Signed-off-by: disqualifier <dev@disqualifier.me>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "mysql"
-version = "0.1.3"
+version = "0.1.4"
 description = "async mysql/mariadb wrapper over aiomysql: two-layer API (friendly verbs + raw escape hatch), fail-loud, config-free"
 requires-python = ">=3.10"
 dependencies = [