feat(xerrors): initial stable release v0.9.0

Structured application errors with typed codes, cause chaining, key-value context fields, and zero-import logz enrichment bridge.

What's included:
- `*Err` type implementing error, errors.Unwrap, json.Marshaler, ErrorCode(), and ErrorContext()
- Twelve typed Code constants aligned with gRPC canonical status names
- New / Wrap factory constructors plus InvalidInput / NotFound / Internal convenience constructors
- Builder methods WithContext and WithError for attaching structured fields and causes
- Duck-typed ErrorCode() / ErrorContext() bridge so logz auto-enriches log records without an import

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

compliance_test.go

@@ -0,0 +1,24 @@
+package xerrors_test
+
+import "code.nochebuena.dev/go/xerrors"
+
+// Compile-time contract verification.
+//
+// These assertions are zero-cost at runtime — the nil pointers are never
+// dereferenced. If any method is removed or its signature changes, the build
+// fails immediately here rather than at a distant call site in another module.
+
+// richError is the shape that logz and other consumers expect from any structured
+// error produced by this package. Adding methods to Err is non-breaking. Removing
+// any of these or changing their signature is a breaking change.
+var _ interface {
+	error
+	Unwrap() error
+	ErrorCode() string
+	ErrorContext() map[string]any
+} = (*xerrors.Err)(nil)
+
+// jsonMarshaler verifies Err implements json.Marshaler.
+var _ interface {
+	MarshalJSON() ([]byte, error)
+} = (*xerrors.Err)(nil)