fix(showcase): break wn-breadcrumb slotchange infinite loop (WHYNOT-WP-0002 T11)

WnBreadcrumb._onSlot inserted separator <span>s into its own light DOM on
slotchange but cleaned up in the shadow DOM, so they were never removed — each
insertion re-fired slotchange, looping the main thread and wedging the showcase
page. Made _onSlot idempotent: exclude own separators when reading items, and
mutate only when separators are not already correct.

- Un-fixme the showcase visual test; add a warm-up full-page capture so
  deviceScaleFactor-2 sub-pixel snapping settles before the assertion. All 5
  visual tests pass.
- Remove the dead Google-Fonts @import from colors_and_type.css (token stacks are
  system-ui; webfont unused + a CI-flake source; no visual change).
- Unblocks WHYNOT-WP-0003 T08 (showcase = per-version visual catalog); both T11
  and T08 marked done.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workplans/WHYNOT-WP-0003-downstream-consumption.md

@@ -225,7 +225,7 @@ anything, complementing the machine-readable manifest.
 
 ```task
 id: WHYNOT-WP-0003-T08
-status: wait
+status: done
 priority: low
 state_hub_task_id: "a0886a4f-cf27-44ef-b8c6-8e61ceda1f84"
 ```
@@ -236,6 +236,13 @@ just to confirm, once T11 lands, that the showcase is deployable/inspectable per
 version (e.g. served from a tag) — no new build, reuse the existing page. Blocked on
 WP-0002-T11.
 
+**Done 2026-06-27.** WP-0002-T11 landed (breadcrumb infinite-loop fixed); the
+showcase page now renders deterministically and is covered by a passing visual test
+(`showcase.png` baseline). No new build — the existing page *is* the per-version
+visual catalog: served statically from any checkout/tag (`pnpm showcase`), it pairs
+with the machine-readable `ir/manifest.json` + `ir/INDEX.md` as the visual side of a
+version's inventory. Nothing further to build.
+
 ---
 
 ## Phase 4 — Deferred: live contract-parity mode (design-only)