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/src/WebBlock.tsx

@@ -11,8 +11,11 @@
 // plain text. Geometry, radius, and fonts mirror CodeBlock/TerminalBlock so a
 // web card reads as one family with them; the whole source list renders inside a
 // fixed-height scroll container (its `.sources` max-height), so a long list
-// scrolls in place rather than growing the card. The list matches what the model
-// saw: the tool already cut it to the source cap, and `truncated` reports that.
+// scrolls in place rather than growing the card. The card draws every source the
+// view carries: the tool already cut the list to its source cap, and `truncated`
+// reports that cut. A content-only transform downstream of the tool — spill-policy
+// replacing an oversized result's text while leaving its presentationMeta whole —
+// can still narrow what the model reads below this list.
 
 import clsx from 'clsx'
 import { MarkdownText } from './markdown/MarkdownText.tsx'