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.

1 files changed, 14 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();
+  });
+});