fix(words): eliminate flicker on block inserter "+" button

The previous pass listened to mousemove ON the content element and
cleared the seam on mouseleave. The "+" button lives at left:-28px
of the content (in the gutter), so the moment the cursor reached the
button it triggered mouseleave on content → seam cleared → button
vanished → cursor was over content again → mousemove → button shown
→ loop. That was the constant flicker.

Three fixes, smallest to largest:

1. **Listen on document, not content.** A single document-level
   mousemove handler with manual bounds checking. No more
   mouseleave/mouseout firing as the cursor crosses element edges.
2. **Hold steady when over the inserter itself.** If `e.target` is
   under `[data-words-block-inserter]`, return early — don't recompute
   or clear the seam. The button can stay reachable forever.
3. **Memoize the seam.** Reuse the existing Seam object whenever the
   snap target (`insertIndex`) is unchanged. Cursor jitter within
   the same snap window no longer triggers re-renders.

Bounds-check buffer: 48px to the left (so the "+" button at -28 is
safely inside the keep-alive zone), 16px on the right, 12px top/bottom.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/uix/eidos/components/words/words-block-inserter.svelte

@@ -88,14 +88,41 @@
 		const content = document.querySelector<HTMLElement>('[data-words-content]');
 		if (!content) return;
 
+		// Track movement on the document, not on `content`, so the "+"
+		// button (which lives in the left margin OUTSIDE the content
+		// box) doesn't fire a `mouseleave` on content the moment the
+		// cursor hits it — that was the source of the flicker loop.
+		// While the cursor is OVER the inserter overlay (button or
+		// line) we hold the current seam steady instead of recomputing
+		// or clearing.
 		function onmove(e: MouseEvent) {
-			seam = findSeamForCursor(e, content);
-		}
-		function onleave() {
-			seam = null;
+			const target = e.target;
+			if (target instanceof Element && target.closest('[data-words-block-inserter]')) {
+				// Cursor is on the inserter itself — keep it visible.
+				return;
+			}
+			const r = content.getBoundingClientRect();
+			// Hide when clearly outside the content gutter (with a small
+			// buffer for the "+" button's left margin).
+			if (
+				e.clientX < r.left - 48 ||
+				e.clientX > r.right + 16 ||
+				e.clientY < r.top - 12 ||
+				e.clientY > r.bottom + 12
+			) {
+				if (seam !== null) seam = null;
+				return;
+			}
+			const next = findSeamForCursor(e, content);
+			// Same snap target → reuse the existing Seam object so Svelte
+			// doesn't re-render the overlay for an identical position.
+			if (next === null && seam === null) return;
+			if (next !== null && seam !== null && next.insertIndex === seam.insertIndex) {
+				return;
+			}
+			seam = next;
 		}
-		content.addEventListener('mousemove', onmove);
-		content.addEventListener('mouseleave', onleave);
+		document.addEventListener('mousemove', onmove);
 
 		const win = content.ownerDocument.defaultView;
 		const onscroll = () => {
@@ -108,8 +135,7 @@
 		win?.addEventListener('resize', onscroll, { passive: true });
 
 		return () => {
-			content.removeEventListener('mousemove', onmove);
-			content.removeEventListener('mouseleave', onleave);
+			document.removeEventListener('mousemove', onmove);
 			win?.removeEventListener('scroll', onscroll);
 			win?.removeEventListener('resize', onscroll);
 		};