feat(core): initial implementation — launcher, logz, xerrors, valid

Introduces `code.nochebuena.dev/einherjar/core` — the foundational implementation
module of the Einherjar framework. Provides four sub-packages that together cover
every service's baseline needs: lifecycle management, structured logging, typed
errors, and struct validation.

- launcher: Launcher interface — three-phase managed lifecycle (OnInit → BeforeStart
  hooks → OnStart → OS signal wait → OnStop in reverse). Accepts
  lifecycle.Component and logging.Logger from contracts. Prints an ASCII art banner
  at startup (EINHERJAR_BANNER=off to suppress). Banner includes core version via
  runtime/debug.ReadBuildInfo() and a loaded-module list for every registered
  component that implements observability.Identifiable. Config struct with
  EINHERJAR_COMPONENT_STOP_TIMEOUT env tag (caarlos0/env syntax, default 15s).

- logz: Logger implementation backed by log/slog. Returns contracts/logging.Logger.
  Detects errs.CodedError and errs.ContextualError (from contracts/errs) to enrich
  log records automatically — replaces the private duck-typed bridge from micro-lib.
  Context helpers: WithRequestID, WithField, WithFields, GetRequestID. Config struct
  with EINHERJAR_LOG_LEVEL (default INFO) and EINHERJAR_LOG_JSON (default false) env
  tags (caarlos0/env syntax); programmatic-only fields StaticArgs and Writer carry no
  tags.

- xerrors: Typed error codes with context enrichment. Complete gRPC canonical set
  (16 codes) plus HTTP 410 ErrGone. Adds ErrOutOfRange, ErrAborted, ErrDataLoss
  over micro-lib. One convenience constructor per code. *Err declares compile-time
  satisfaction of errs.CodedError and errs.ContextualError.

- valid: Struct validation wrapping go-playground/validator/v10. Validator interface
  + MessageProvider interface with full built-in tag coverage (~150 tags) in both
  DefaultMessages (English) and SpanishMessages (Spanish). Backend fully hidden;
  returns *xerrors.Err with ErrInvalidInput or ErrInternal. FieldLevel interface
  abstracts the backend's field-level access for custom validators.
  WithCustomValidator registers custom validation tags at construction time;
  OverrideProvider chains a tag→handler map with a fallback MessageProvider for
  custom tag messages without re-implementing built-ins.

Compliance test enforces CT-6 (at most one exported TypeSpec per file via AST) and
verifies behavioural correctness of all four sub-packages, including custom validator
registration and OverrideProvider composition. Compile-time var _ assertions prove
interface satisfaction.

docs: ADR-001 (core module composition), ADR-002 (logz contracts/errs adoption),
ADR-003 (Config naming convention and caarlos0/env tag standard)

logz/context.go

@@ -0,0 +1,40 @@
+package logz
+
+import "context"
+
+type ctxRequestIDKey struct{}
+type ctxExtraFieldsKey struct{}
+
+// WithRequestID adds a request correlation ID to the context.
+func WithRequestID(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, ctxRequestIDKey{}, id)
+}
+
+// GetRequestID retrieves the correlation ID from the context.
+// Returns "" if not present or if ctx is nil.
+func GetRequestID(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	id, _ := ctx.Value(ctxRequestIDKey{}).(string)
+	return id
+}
+
+// WithField adds a single key-value pair to the context for logging.
+func WithField(ctx context.Context, key string, value any) context.Context {
+	return WithFields(ctx, map[string]any{key: value})
+}
+
+// WithFields adds multiple key-value pairs to the context for logging.
+// Merges with any existing fields — does not overwrite the whole map.
+func WithFields(ctx context.Context, fields map[string]any) context.Context {
+	existing, _ := ctx.Value(ctxExtraFieldsKey{}).(map[string]any)
+	newMap := make(map[string]any, len(existing)+len(fields))
+	for k, v := range existing {
+		newMap[k] = v
+	}
+	for k, v := range fields {
+		newMap[k] = v
+	}
+	return context.WithValue(ctx, ctxExtraFieldsKey{}, newMap)
+}