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/doc.go

@@ -0,0 +1,27 @@
+// Package logz provides a structured, leveled logger backed by the standard
+// library's log/slog package.
+//
+// logz.New returns a logging.Logger (from contracts). No concrete type is
+// exported — callers depend on the interface, not the implementation.
+//
+// Error enrichment: when an error passed to Logger.Error implements
+// errs.CodedError or errs.ContextualError (from contracts/errs), the
+// corresponding fields are automatically appended to the log record without
+// either package importing the other.
+//
+// Context enrichment: WithRequestID, WithField, and WithFields store values
+// in context.Context. Logger.WithContext reads them back and attaches
+// request_id and any extra fields to every subsequent log record.
+//
+// Usage:
+//
+//	logger := logz.New(logz.Options{
+//	    Level:      slog.LevelDebug,
+//	    JSON:       true,
+//	    StaticArgs: []any{"service", "api"},
+//	})
+//
+//	ctx = logz.WithRequestID(ctx, requestID)
+//	reqLogger := logger.WithContext(ctx)
+//	reqLogger.Info("handling request", "path", r.URL.Path)
+package logz