feat(desktop): artifact evolution chain + Board/Timeline views

Adds a version-evolution capability to the Artifacts pane:
  - Evolution chain: LCS-based diff across successive versions of the
    same logical artifact; when a blob is missing, the chain honestly
    surfaces a placeholder rather than fabricating diff content.
  - Board / Timeline tab switcher on the Artifacts page.
  - Auto-grouping into 5 kinds so the Board reads at a glance.

Files:
  src/renderer/artifacts-board.js       — new Board / Timeline / Evolution views
  src/renderer/artifacts.js             — wire Board tab + evolution rail
  src/renderer/index.html               — script tag + tab markup
  src/renderer/style.css                — tail append (~200 lines)
  test/artifact-evolution-board.test.js — 23 new tests
  docs/artifact-board-fixture.json      — fixture seed
  docs/qa-artifact-evolution/           — 3 QA screenshots + fixture.html
  scripts/qa-cdp-shoot-artifact-v2.mjs  — QA capture harness

Merged as source-repo test-real @ 7279870 (with lane-side conflict
resolution against test-real @ c1d93aa: style.css tail-append union;
independent of Lanes C/A/D CSS sections).

Test suite: 1727/1727 pass (+23 over Lane C+A+D baseline of 1704).

Follow-up L-3 (in upstream ledger): runtime ArtifactServer to grow
a snapshot store so the "missing blob" fallback becomes rare.

examples/desktop/src/renderer/index.html

@@ -1324,6 +1324,10 @@
        from any tab. -->
   <script src="./fork-compare.js"></script>
   <script src="./tool-cards.js"></script>
+  <!-- artifacts-board.js defines window.__dshArtifactsBoard (Board/Timeline/
+       Evolution renderers) and MUST load before artifacts.js so the L0 row
+       version-chip click handler + view switcher find the module ready. -->
+  <script src="./artifacts-board.js"></script>
   <script src="./artifacts.js"></script>
   <script src="./plugin-runtime-fold.js"></script>
   <script src="./mcp-tool-name.js"></script>