Return sink write errors from every handler (closes #22)

The console and JSON handlers threw away the error from their write to
stdout, so a lost log line looked delivered. They now return it,
wrapped. The webhook handler also returns an error for a status outside
2xx. The multiplex handler passes the record to every handler and
returns their errors joined with errors.Join instead of stopping at the
first.

Both stdout handlers gain an unexported writer, nil meaning os.Stdout at
write time, so tests can supply a sink that fails. The README says what
each handler returns and that the slog.Logger methods discard a
handler's error.

Model: opus-5-5

TODO.md

@@ -24,6 +24,10 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
 
 # Completed Steps
 
+* 2026-10-06: every handler now returns a failed delivery from Handle
+    instead of discarding it: console and JSON return the stdout write
+    error, the webhook also fails on a non-2xx answer, and the multiplex
+    delivers to every handler and returns their errors joined
 * 2026-08-10: fixed every handler discarding slog attributes: console,
     JSON and webhook handlers now emit record attributes, accumulate
     WithAttrs without mutating the receiver, and honour WithGroup;