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)

xerrors/doc.go

@@ -0,0 +1,29 @@
+// Package xerrors provides structured application errors with stable typed codes,
+// cause chaining, and key-value context fields.
+//
+// Every error carries a machine-readable Code (gRPC-aligned wire value), a
+// human-readable message, an optional cause, and optional structured fields.
+// *Err implements errs.CodedError and errs.ContextualError from contracts,
+// enabling logz to enrich log records automatically without importing this package.
+//
+// Usage:
+//
+//	// Named constructors for common codes
+//	err := xerrors.NotFound("user %s not found", userID)
+//	err := xerrors.InvalidInput("email is required")
+//
+//	// Builder pattern for structured context
+//	err := xerrors.New(xerrors.ErrInvalidInput, "validation failed").
+//	    WithContext("field", "email").
+//	    WithContext("rule", "required").
+//	    WithError(cause)
+//
+//	// Inspecting errors
+//	var e *xerrors.Err
+//	if errors.As(err, &e) {
+//	    switch e.Code() {
+//	    case xerrors.ErrNotFound:
+//	        // handle 404
+//	    }
+//	}
+package xerrors