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>

keypress/cbreak_other.go

@@ -0,0 +1,19 @@
+// SPDX-FileCopyrightText: (C) Copyright 2026 Pim van Pelt <pim@ipng.ch>
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build !linux && !openbsd && !freebsd && !netbsd && !dragonfly && !darwin
+
+package keypress
+
+import "errors"
+
+// termiosState is a placeholder so cbreak/restore have a consistent signature on
+// platforms without a supported termios path. On those platforms cbreak always
+// fails, so WaitForKey degrades to waiting on the context (it never reads stdin).
+type termiosState struct{}
+
+func cbreak(int) (*termiosState, error) {
+	return nil, errors.New("keypress: cbreak is unsupported on this platform")
+}
+
+func restore(int, *termiosState) error { return nil }