Test palm rejection for add-page button taps

Move the draw-or-navigate choice into touchIntent() so the rule the
canvas uses can be tested, and cover finger taps on the plus button:
accepted after writing and while the pen hovers, rejected while the
pen is down, for palm-sized contacts and right after pen activity.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GaoZtJQZwLXkEgWs8twCia

src/lib/ink/palm.ts

@@ -27,6 +27,15 @@ const PALM_CONTACT_PX = 35;
 /** What an accepted touch would do: ink (draw/erase) or pan/zoom. */
 export type TouchIntent = 'draw' | 'navigate';
 
+/**
+ * Which policy a touch falls under. It only draws when finger ink is on and it
+ * didn't land on a button; taps on buttons (e.g. add page) navigate, so they keep
+ * working between pen strokes.
+ */
+export function touchIntent(fingerInks: boolean, onButton: boolean): TouchIntent {
+	return fingerInks && !onButton ? 'draw' : 'navigate';
+}
+
 export interface Decision {
 	accept: boolean;
 	reason: string;