feat: Validate + keypress subpackage; RFC-style design.md (v1.2.0)

Validate(root): optional startup/test check for tree authoring faults —
>1 slot child per node, empty word, duplicate sibling words, dead-end
node — traversing circular slots without looping (#3).

keypress subpackage: WaitForKey(ctx, cancel) cancels a context on any
keystroke for watch-style streaming commands, with per-GOOS cbreak
(linux TCGETS/TCSETS, BSD TIOCGETA/TIOCSETA) and a non-tty/unsupported
fallback that just waits on ctx. Lifts the last OpenBSD-specific bit out
of evpnc/maglevc's watch.go (#6).

docs: replace PROPOSAL.md with an RFC-2119 design.md (FR/NFR for the
library). Example now dogfoods Validate (a unit test) and keypress (a
bounded `watch` command).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

example/main.go

@@ -35,6 +35,7 @@ import (
 	"strings"
 
 	cli "git.ipng.ch/ipng/golang-cli"
+	"git.ipng.ch/ipng/golang-cli/keypress"
 )
 
 // inventory is the "client" C that the tree is generic over. In a real app this
@@ -154,6 +155,28 @@ func runColors(context.Context, inventory, []string) error {
 	return nil
 }
 
+// runWatch streams a few simulated events, stopping early if a key is pressed.
+// In a real CLI this would be a server-side stream; here it is a bounded loop so
+// the demo never hangs when stdin is not a terminal. Press any key (in a TTY) to
+// stop it before it finishes.
+func runWatch(ctx context.Context, _ inventory, _ []string) error {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	go keypress.WaitForKey(ctx, cancel)
+
+	for i := 1; i <= 5; i++ {
+		select {
+		case <-ctx.Done():
+			return nil // key pressed
+		default:
+		}
+		if err := cli.Emit(cli.KV("event", fmt.Sprintf("%d", i)), map[string]any{"event": i}); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func runQuit(context.Context, inventory, []string) error { return cli.ErrQuit }
 
 // buildTree is the single source of truth for the command set, built with the
@@ -168,6 +191,7 @@ func buildTree() *cli.Node[inventory] {
 						b.Slot("<svc>", "show one service", dynServices, runShowServerService))))),
 		b.Dir("ping", "ping a server",
 			b.Slot("<name>", "ping this server", dynServers, runPing)),
+		b.Cmd("watch", "stream a few events (any key stops it)", runWatch),
 		b.Cmd("colors", "show the ANSI color palette", runColors),
 		b.Cmd("quit", "exit the shell", runQuit),
 		b.Cmd("exit", "exit the shell", runQuit),