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, 2 insertions, 2 deletions

diff --git a/src/features/layout/ui/TileLeaf.svelte b/src/features/layout/ui/TileLeaf.svelte
index 2d4a91b..0fd6430 100644
--- a/src/features/layout/ui/TileLeaf.svelte
+++ b/src/features/layout/ui/TileLeaf.svelte
@@ -11,7 +11,7 @@
    */
   import type { TileMarker } from "@dispatch/layout-contract";
   import { viewRegistry, type RegisteredView } from "../logic/registry";
-  import { markerOutline } from "../logic/marker";
+  import { MARKER_OUTLINE_OFFSET, markerOutline } from "../logic/marker";
   import FallbackTile from "./FallbackTile.svelte";
 
   let {
@@ -38,7 +38,7 @@
 {/snippet}
 
 {#if outline !== undefined}
-  <div style:outline={outline}>{@render content()}</div>
+  <div style:outline={outline} style:outline-offset={MARKER_OUTLINE_OFFSET}>{@render content()}</div>
 {:else}
   {@render content()}
 {/if}