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_test.go

@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: (C) Copyright 2026 Pim van Pelt <pim@ipng.ch>
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"testing"
+
+	cli "git.ipng.ch/ipng/golang-cli"
+)
+
+// TestTreeValid keeps the example's command tree free of the authoring faults
+// Validate checks for. It doubles as the recommended way to use Validate: from
+// a unit test, so a malformed tree fails the build rather than misdispatching
+// at runtime.
+func TestTreeValid(t *testing.T) {
+	if err := cli.Validate(buildTree()); err != nil {
+		t.Fatalf("buildTree() has authoring faults:\n%v", err)
+	}
+}