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>

README.md

@@ -13,7 +13,7 @@ and emit; their records flow into the handlers `log_setup` wired.
 ## Install
 
 ```
-log_setup @ git+ssh://git@git.rethinkstudios.io/rethink-public/log_setup.git@v0.3.0
+log_setup @ git+ssh://git@git.rethinkstudios.io/rethink-public/log_setup.git@v0.3.1
 ```
 
 No dependencies — stdlib only.