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>

README.md

@@ -113,6 +113,35 @@ Both are additive conveniences: every builder method returns a plain
 `*cli.Node[C]`, so builder and struct-literal construction interoperate, and you
 can still drive the lifecycle yourself with `Shell`/`Dispatch` instead of `App`.
 
+`cli.Validate(root)` reports common authoring faults (more than one slot child
+under a node, an empty word, duplicate sibling words, a dead-end node). It is
+optional; the idiomatic use is a one-line unit test so a malformed tree fails the
+build:
+
+```go
+func TestTreeValid(t *testing.T) {
+    if err := cli.Validate(buildTree()); err != nil { t.Fatal(err) }
+}
+```
+
+## Streaming commands
+
+For a `watch`-style command that streams until interrupted, the
+[`keypress`](keypress) subpackage stops it on any keystroke:
+
+```go
+func runWatch(ctx context.Context, c Client, _ []string) error {
+    ctx, cancel := context.WithCancel(ctx)
+    defer cancel()
+    go keypress.WaitForKey(ctx, cancel) // any key cancels ctx
+    stream, _ := c.Watch(ctx, req)
+    for { ev, err := stream.Recv(); /* returns when ctx is cancelled */ }
+}
+```
+
+When stdin is not a terminal it simply waits on the context, so piped/one-shot
+use never blocks on a keypress.
+
 ## Output: color and JSON
 
 A command describes its result **once** and the framework renders it: