fix(sw): square the multi step header process ends

- SWProcessHeader appearance="multi": the process base and both end
  covers meet the file name and the window area with right angles
  (border-radius 0, multi block only). default keeps the Strana top
  corners 6 / covers 5; step tabs, arrows, colors and sizes unchanged
- DESIGN_RULES §14 and Step Header spec §9.2 record the multi-only rule
- process-header-states.check.mjs: default 6 / 5 kept, multi 0 (17/17);
  build:sw and check:ai-docs pass, Storybook zoom shows square multi ends

packages/sw/tests/process-header-states.check.mjs

@@ -77,6 +77,14 @@ const GROUPS = [
   check("styles.css: the empty label keeps its color on hover (no :not(:hover) scope)",
     /\[data-document-empty\] \.hm-sw-header__document-button \.hm-sw-header__document-name ?\{ ?color: ?var\(--hm-sw-process-document-empty-color\)/.test(css)
     && !/\[data-document-empty\] \.hm-sw-header__document-button:not\(:hover\)/.test(css))
+  // 6. Process-area ends (2026-11-05): `multi` square (base and end covers radius 0), `default` keeps the Strana 6 / 5.
+  const block = selector => css.match(new RegExp(`(?:^|\\}|\\*/) ?${selector.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")} ?\\{([^}]*)\\}`))?.[1] ?? null
+  const base = block(".hm-sw-process-bar::before") ?? ""
+  check("styles.css: default keeps the process base top corners 6 and the end covers 5",
+    /border-radius: ?6px 6px 0 0/.test(base) && /border-top-left-radius: ?5px/.test(block('.hm-sw-process-bar__rest[data-side="start"]') ?? "")
+    && /border-top-right-radius: ?5px/.test(block('.hm-sw-process-bar__rest[data-side="end"]') ?? ""), { base })
+  check("styles.css: multi sets radius 0 on the process base and both end covers",
+    /\.hm-sw-process-bar\[data-appearance="?multi"?\]::before ?, ?\.hm-sw-process-bar\[data-appearance="?multi"?\] ?> ?\.hm-sw-process-bar__rest ?\{ ?border-radius: ?0 ?;? ?\}/.test(css))
 }
 
 const passed = results.filter(Boolean).length