fix(docs): clarify SSR remote fallback behavior

[dmchoi77]

Summary

Clarify documentation that could imply an ErrorBoundary fallback is always rendered in the server's initial HTML when a remote load fails. The docs now separate the responsibilities of the route loader, SSR rendering, and client/hydration recovery so the guaranteed behavior during an SSR outage is explicit.

Context

• The previous wording said that an unavailable remote renders the boundary fallback, but React SSR does not guarantee that a rejected lazy module causes an ErrorBoundary to switch to its fallback.
• SSR may produce a recoverable error or switch to client rendering, while the fallback may be handled during client rendering or hydration recovery.
• The route loader therefore owns route-level isolation for failed remote imports and stylesheet lookup, while RemoteBoundary owns client-side error presentation and recovery.

Changes

• Updated the server-rendering example comment in packages/tanstack/README.md to describe route-loader failure isolation.
• Documented that an SSR ErrorBoundary fallback is not guaranteed in the initial HTML.
• Separated lazyRemote retry behavior from SSR ErrorBoundary fallback behavior.
• Updated apps/README.md to distinguish client-side fallback behavior from SSR initial-HTML behavior during an outage.
• No runtime code, public API, Federation plugin configuration, or fallback implementation was changed.

Validation

• pnpm exec prettier --check packages/tanstack/README.md apps/README.md: passed
• Runtime/build tests: not run; this commit changes documentation only

[pkg-pr-new]

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npm i https://pkg.pr.new/module-federation/tanstack/@module-federation/tanstack@7

commit: c27e246