feat(smtp): initial implementation — SMTP client with no-op fallback and template rendering (v1.0.0)

Introduces code.nochebuena.dev/einherjar/smtp — the email delivery starter for the
Einherjar framework. New module with no micro-lib counterpart. Pure stdlib — no
external dependencies beyond contracts and core.

Interfaces (CT-6: one TypeSpec per file):
- Sender — Send(ctx context.Context, msg Message) error
- Component — lifecycle.Component + observability.Checkable + Sender
- Message — To, CC, BCC, ReplyTo, Subject, Body, ContentType, Attachments
- Attachment — Name, ContentType string; Data io.Reader (consumed once on Send)
- Template — wraps html/template for email rendering

Implementation:
- New(logger, cfg) Component — returns noopClient when cfg.Host is empty;
  SMTP absence must never block a transaction (e.g. account creation)
- noopClient: all lifecycle and Send methods return nil; Send logs Warn;
  Priority LevelDegraded; Name "smtp"
- smtpClient: OnInit/OnStart/OnStop are no-ops (stateless stdlib client);
  HealthCheck: TCP dial to Host:Port; Priority LevelDegraded
- Send: buildRawMessage → netsmtp.SendMail; BCC passed in SMTP envelope
  (RCPT TO) but never written to message headers (RFC 5321/5322 compliance)
- buildRawMessage: plain message without attachments; multipart/mixed with
  quoted-printable body + base64 attachments when Attachments non-empty;
  multipart.NewWriter created first to obtain boundary before headers are written
- ParseFS(fsys, patterns...) (*Template, error) — wraps html/template.ParseFS
- Template.Render(name, data) (string, error) — executes named template
- net/smtp aliased as netsmtp to avoid package-name collision in package smtp

Config (EINHERJAR_SMTP_* env vars):
  Host, Port(587), User, Password, From

- Component interface embeds observability.Identifiable; identifiable.go implements
  ModulePath and ModuleVersion on both smtpClient and noopClient via
  runtime/debug.ReadBuildInfo() — prints in launcher banner

doc.go

@@ -0,0 +1,52 @@
+// Package smtp provides a lifecycle-managed SMTP client for sending email
+// in Einherjar applications.
+//
+// # Overview
+//
+// [New] returns a [Component] that satisfies [lifecycle.Component] lifecycle
+// hooks, [observability.Checkable] with degraded priority, and the [Sender]
+// interface for dispatching email. When [Config.Host] is empty, [New] returns
+// a no-op implementation that logs a warning and silently discards every message —
+// email failure must never block a transaction.
+//
+// # Lifecycle Registration
+//
+//	client := smtp.New(logger, cfg)
+//	launcher.Register(client)
+//	health.Register(client)
+//
+// # Sending
+//
+// Services depend only on [Sender]. Build a [Message] with a pre-rendered body,
+// then call Send:
+//
+//	msg := smtp.Message{
+//	    To:          []string{"alice@example.com"},
+//	    Subject:     "Your receipt",
+//	    Body:        rendered,     // pre-rendered string from Template.Render
+//	    ContentType: "text/html",
+//	}
+//	if err := mailer.Send(ctx, msg); err != nil {
+//	    return err
+//	}
+//
+// # Template Rendering
+//
+// [ParseFS] wraps stdlib html/template for HTML email rendering. Rendering is
+// intentionally separate from Send — callers render to a string and assign it
+// to Message.Body. This keeps Send unit-testable without template involvement.
+//
+//	tmpl, err := smtp.ParseFS(os.DirFS("templates"), "*.html")
+//	body, err := tmpl.Render("receipt.html", data)
+//	msg := smtp.Message{Body: body, ContentType: "text/html", ...}
+//
+// # Configuration
+//
+// All fields are read from environment variables with the EINHERJAR_SMTP_* prefix:
+//
+//   - EINHERJAR_SMTP_HOST     — SMTP server hostname (empty = no-op mode)
+//   - EINHERJAR_SMTP_PORT     — default: 587 (STARTTLS)
+//   - EINHERJAR_SMTP_USER     — auth username (optional)
+//   - EINHERJAR_SMTP_PASSWORD — auth password (optional)
+//   - EINHERJAR_SMTP_FROM     — envelope sender address
+package smtp