fix: JSON extra cannot clobber canonical time/level/module fields (v0.3.1)

guard the extra-merge loop with the formatter's own output keys (time/level/module/
message). stdlib LogRecord rejects extra keys colliding with real attribute names, but
time/level are NOT LogRecord attrs, so a caller's extra={"time":...}/{"level":...}
previously overwrote the UTC timestamp / levelname — the two fields Loki/Grafana alert
on. now those keys are reserved and a colliding extra is dropped.

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

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "log_setup"
-version = "0.3.0"
+version = "0.3.1"
 description = "stdlib app-entry-point logging setup: live run.log, rotation, gzip, retention, consistent format"
 requires-python = ">=3.10"
 dependencies = []