v0.1.12 SetConsoleWriter routes console output to a host writer

A host that draws on the terminal (a TUI) cannot let the logger write to
stdout or stderr, yet the records are worth showing inside the host.
SetErrorHandler already covers internal diagnostics; SetConsoleWriter is
its sibling for console output.

- SetConsoleWriter(w) sends console output of every console_target to w,
  split's warn-and-above records included; nil restores stdout/stderr.
  It holds across ApplyConfig and is safe while logging.
- One stored value: ApplyConfig and SetConsoleWriter publish the console
  writers through storeConsole; the processor's split branch now reads
  the stored warn writer instead of writing os.Stderr directly.
- TestLoggerConsoleTargets becomes TestConsoleWriterTakesEveryTarget,
  which asserts where each target's records land; a second test checks
  that nil restores the configured writers.
- README names both hooks for terminal hosts; the API doc's callback
  contract covers console writers.

Verified: go build ./... ok; gofmt -l on changed files clean; go vet ./...
ok; go test ./... ok (all four packages, under 1 s); go test -race on the
new tests ok; mutation run 6 of 6 caught.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyaMAhyEzAp3ULLWJZG4jQ

doc/api.md

@@ -390,7 +390,7 @@ Logger methods support concurrent calls. Lifecycle and configuration changes are
 
 `ApplyConfig` takes a snapshot. Emission copies the argument slice, and `LogStructured` also copies the top-level fields map. Referenced values (byte slices, nested maps/slices, pointers, Stringers, and custom marshalers) remain caller-owned: keep them immutable until a successful `Flush` or `Stop`, or pass an application-created snapshot. Do not mutate configuration while an `ApplyConfig` call is reading it.
 
-Builders and standalone formatter/sanitizer configuration are not synchronized; finish configuration before sharing. `SetSpawn` callbacks must launch the function asynchronously and return promptly. Error handlers must return promptly and must not invoke lifecycle/configuration methods or `Flush`.
+Builders and standalone formatter/sanitizer configuration are not synchronized; finish configuration before sharing. `SetSpawn` callbacks must launch the function asynchronously and return promptly. Error handlers and `SetConsoleWriter` writers must return promptly and must not invoke lifecycle/configuration methods or `Flush`.
 
 ### Usage Pattern Example