fix: check the meta key budget before scanning every key

The key-count cap exists to catch a whole request body dumped into meta.
That is exactly the shape where the per-key regex runs tens of thousands
of times before the real reason surfaces, so the cheap check goes first.

Also correct two stale docstrings: postgres.py still claimed 22 columns
(it is 24), and _canonical_meta_json promised to raise on non-finite
floats. It is evaluated inside _record's degradation try, so the real
outcome is a warning plus a dropped row -- never an error the caller
sees. What the gate actually buys us is the SQLite side, whose meta is a
TEXT column that would happily store a literal NaN.

src/polygateway/types.py

@@ -115,6 +115,10 @@ def _validate_tenant_id(tenant_id: str | None, origin: str) -> None:
 
 def _validate_meta_keys(meta: Mapping[str, Any], origin: str) -> None:
     """键形态与数量;键集合被假定为低基数且稳定,故收紧到 OTel semconv 字符集。"""
+    # 数量闸先于逐键校验: 这道闸要防的正是"整个请求体被塞进 meta"的形态,
+    # 那时逐键正则会先跑上万次才报出真正的原因,拖慢的恰是出错路径
+    if len(meta) > _META_MAX_KEYS:
+        raise ValueError(f"{origin} 的 meta 键数超限(上限 {_META_MAX_KEYS}): {len(meta)}")
     for key in meta:
         if not isinstance(key, str):
             raise ValueError(f"{origin} 的 meta 键必须是 str: {key!r}")
@@ -127,8 +131,6 @@ def _validate_meta_keys(meta: Mapping[str, Any], origin: str) -> None:
             raise ValueError(
                 f"{origin} 的 meta 键 {key!r} 不合法: 只允许小写字母/数字/下划线/点,长度 1-64"
             )
-    if len(meta) > _META_MAX_KEYS:
-        raise ValueError(f"{origin} 的 meta 键数超限(上限 {_META_MAX_KEYS}): {len(meta)}")
 
 
 def _validate_meta_values(meta: Mapping[str, Any], origin: str) -> None: