docs(client-web): sync the web card contracts to the scrolling source list

The READMEs of ui-primitives and ui-conversation still documented the
removed maxSources prop, DEFAULT_WEB_MAX_SOURCES, CHAT_WEB_MAX_SOURCES,
and the head/tail collapse; the 2026-07-30 frontend note described the
same as current behavior. Restate all of them as the fixed-height scroll
container and cross-link the two notes both ways.

Bound the model-visible/card-visible correspondence: spill-policy's
tools/post-execute replaces an oversized result's content while leaving
presentationMeta whole, so the card's contract is the view it receives,
not the model's context.

Narrow the unit test's name and header comment to what jsdom establishes
(one <ol>, no expand control); jsdom does not resolve CSS Modules layout,
so the scroll geometry belongs to an assembled browser case.

packages/client/ui-primitives/tests/web-block.spec.tsx

@@ -1,8 +1,8 @@
 // @vitest-environment jsdom
 // WebBlock: both kinds of the web card. The search card's answer, its citation
 // list with the title-or-hostname label fallback and optional snippet/date, the
-// full source list rendered inside a scroll container, and the truncated
-// indicator; the fetch card's linked URL, status, and truncation. Safe-link
+// full source list under one <ol>, and the truncated indicator; the fetch
+// card's linked URL, status, and truncation. Safe-link
 // attributes on both kinds: an http(s) URL becomes an external anchor
 // (target/rel), any other URL renders as plain text with no href.
 
@@ -123,10 +123,12 @@ describe('WebBlock search card', () => {
     expect(off.queryByText('来源列表已截断')).toBeNull()
   })
 
-  it('renders every source with no expand control, in one scroll container', () => {
-    // The card shows the whole list the tool returned — the same sources the
-    // model saw — with no head/tail collapse and no expand button; a long list
-    // scrolls within the .sources container instead.
+  it('renders every source in one <ol> with no expand control', () => {
+    // The card shows the whole list the tool returned, with no head/tail
+    // collapse and no expand button. jsdom does not resolve the CSS Modules
+    // layout, so the scroll geometry the `.sources` max-height produces is
+    // pinned by the assembled browser case in apps/web/tests/web-search-round.e2e.ts,
+    // not here.
     const view = render(<WebBlock kind="search" sources={sources(30)} truncated={false} />)
     expect(view.container.querySelectorAll('li[class^="_source_"]')).toHaveLength(30)
     expect(view.container.querySelector('[aria-expanded]')).toBeNull()