feat(logz): initial stable release v0.9.0

Structured logger backed by log/slog with request-context enrichment, extra-field context helpers, and duck-typed automatic error enrichment.

What's included:
- `Logger` interface with Debug / Info / Warn / Error / With / WithContext; `New(Options)` constructor writing to os.Stdout
- `WithRequestID` / `GetRequestID` and `WithField` / `WithFields` context helpers — package owns both context keys
- Automatic error_code and context-field enrichment in Logger.Error via duck-typed errorWithCode / errorWithContext interfaces (no xerrors import)

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

compliance_test.go

@@ -0,0 +1,7 @@
+package logz_test
+
+import "code.nochebuena.dev/go/logz"
+
+// Verify New returns a Logger — the concrete slogLogger type satisfies the interface.
+// If any method is removed or its signature changes, this file fails to compile.
+var _ logz.Logger = logz.New(logz.Options{})