fix(layout): draw diagnostic tile markers inset + frame the sidebar viewport, not its scrolled content

Two renderer bugs made the diagnostic layout plugin's borders useless:

1. Borders went past the panels and were invisible at screen edges: CSS
   outlines draw OUTSIDE the border box by default, and tiles span their
   containers edge-to-edge, so the 4px ring overflowed the visible area
   and was clipped. The outline is now inset (outline-offset: -4px) so
   it draws inside the tile's box.

2. The green side border scrolled with the sidebar: the marker was drawn
   on the TilePanels root, which is the SCROLLED CONTENT of the split's
   collapsible child. The marker now lives on the split's scroll-viewport
   div — the sidebar's fixed, opaquely-painted frame: its box never moves
   (only its contents scroll), and its own outline is never clipped by its
   overflow. (The aside behind it can't carry the marker — the scroll
   viewport's bg-base-100 paints over an outline drawn on the aside.)

markerStyle() centralizes the full declaration (outline + inset offset)
and returns undefined for unmarked/unknown markers, so unmarked trees
still render with zero outline CSS (pinned by the existing tests).

Paint layering verified with a headless-Chromium screenshot of a probe
page reproducing the exact DOM + computed styles.

2 files changed, 35 insertions, 1 deletions

diff --git a/src/features/layout/logic/marker.test.ts b/src/features/layout/logic/marker.test.ts
index d9e3826..53b91b0 100644
--- a/src/features/layout/logic/marker.test.ts
+++ b/src/features/layout/logic/marker.test.ts
@@ -1,5 +1,6 @@
+import type { TileMarker } from "@dispatch/layout-contract";
 import { describe, expect, it } from "vitest";
-import { markerOutline } from "./marker";
+import { markerOutline, markerStyle } from "./marker";
 
 describe("markerOutline — the diagnostic tile-border mapping", () => {
   it("main → a 4px solid red outline (the main tile's marker)", () => {
@@ -14,3 +15,15 @@ describe("markerOutline — the diagnostic tile-border mapping", () => {
     expect(markerOutline(undefined)).toBeUndefined();
   });
 });
+
+describe("markerStyle — the complete inline style (outline + inset offset)", () => {
+  it("insets the outline so it draws INSIDE the tile's box (never clipped at screen edges)", () => {
+    expect(markerStyle("main")).toBe("outline: 4px solid red; outline-offset: -4px");
+    expect(markerStyle("side")).toBe("outline: 4px solid green; outline-offset: -4px");
+  });
+
+  it("no/unknown marker → NO style at all (unmarked nodes carry zero outline CSS)", () => {
+    expect(markerStyle(undefined)).toBeUndefined();
+    expect(markerStyle("purple" as unknown as TileMarker)).toBeUndefined();
+  });
+});
diff --git a/src/features/layout/logic/marker.ts b/src/features/layout/logic/marker.ts
index eb1ff84..46259e0 100644
--- a/src/features/layout/logic/marker.ts
+++ b/src/features/layout/logic/marker.ts
@@ -10,6 +10,12 @@
  * (not a border) is used so the diagnostic draws WITHOUT changing any
  * tile's layout/size (4px solid, clearly visible).
  *
+ * The outline is INSET (`outline-offset: -4px`) so it draws INSIDE the
+ * tile's box: tiles span their containers edge-to-edge, so the default
+ * (outside) outline overflows the visible area and is clipped away at the
+ * screen edges. The views inside the tiles have no opaque full-bleed
+ * background, so the inset ring stays on top of the tile's content.
+ *
  * The marker is purely presentational: unknown/absent → no outline
  * (existing trees render exactly as before), and no other behavior is
  * attached to it anywhere in the renderer.
@@ -23,3 +29,18 @@ export function markerOutline(marker: TileMarker | undefined): string | undefine
   if (marker === "side") return "4px solid green";
   return undefined;
 }
+
+/** The inset that keeps the outline INSIDE the tile's box (see the header note). */
+export const MARKER_OUTLINE_OFFSET = "-4px";
+
+/**
+ * The COMPLETE inline style for a marked element — outline + inset offset in
+ * one declaration — or `undefined` when unmarked/unknown, so unmarked nodes
+ * carry no outline CSS at all (existing trees render exactly as before).
+ */
+export function markerStyle(marker: TileMarker | undefined): string | undefined {
+  const outline = markerOutline(marker);
+  return outline === undefined
+    ? undefined
+    : `outline: ${outline}; outline-offset: ${MARKER_OUTLINE_OFFSET}`;
+}