Resolve relay `emit` only when relaying

`supervise_run_process()` bound the relay `emit` method
(`getattr(log, relay_level)`) unconditionally at entry, so a
bad/typo'd `relay_level` raised `AttributeError` even for a
call that requested NO relay (the common silent path). Bind
it lazily behind `relay_stdout or relay_stderr` so only an
actual relay request validates the level.

Review: PR #465 (copilot-pull-request-reviewer)
https://github.com/goodboy/tractor/pull/465#pullrequestreview-4548191641

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/trionics/_subproc.py

@@ -190,7 +190,14 @@ async def supervise_run_process(
             )
 
     '''
-    emit: Callable[[str], None] = getattr(log, relay_level)
+    # resolve the relay emit-method ONLY when actually relaying so
+    # a bad/typo'd `relay_level` can't crash a NON-relay call (and
+    # we don't bind an unused method on the common silent path).
+    emit: Callable[[str], None]|None = (
+        getattr(log, relay_level)
+        if (relay_stdout or relay_stderr)
+        else None
+    )
     tag: str = (
         label
         or