fix: convert sync loader errors to rejected promises

[uhyo]

When a loader throws a synchronous error, the exception previously
propagated through the Router's useMemo, crashing the entire
component. This converts sync errors to rejected promises so they
behave identically to async loader errors — surfaced via React's
use() hook and catchable by Error Boundaries.

Also adds FAQ documentation recommending the root layout error
boundary pattern for handling loader errors.

https://claude.ai/code/session_01PDeppPqxy1CVE8oWJFiHLM

[Copilot]

Pull request overview

Fixes Router crashes caused by synchronous loader exceptions by preventing errors from escaping the Router’s useMemo, and instead re-throwing them during route rendering so React Error Boundaries can handle them. Also adds tests and documentation guidance for error-boundary placement with loaders.

Changes:

• Wrap synchronous loader exceptions in a cached sentinel (LoaderError) instead of letting them throw during loader execution.
• Re-throw wrapped synchronous loader errors during RouteRenderer render to enable Error Boundary handling.
• Add loader error tests and extend loader docs with an error-handling FAQ section.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
packages/router/src/core/loaderCache.ts	Catches sync loader throws and caches a wrapper sentinel instead of throwing through Router memoization.
packages/router/src/Router/RouteRenderer.tsx	Detects the wrapper sentinel and throws the original error during render for Error Boundaries.
packages/router/src/tests/loader.test.tsx	Adds coverage ensuring sync loader errors are handled by Error Boundaries (root and nested).
packages/docs/src/pages/LearnLoadersPage.tsx	Documents recommended Error Boundary placement for loader error handling.

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[Copilot]

PR title/description say sync loader errors are converted to rejected promises, but this change instead caches a LoaderError sentinel and relies on RouteRenderer to throw during render. To avoid confusion for reviewers/users, either update the PR metadata (and any docs wording) to match this implementation, or change the cache value here to a rejected Promise (and handle it consistently downstream).

[Copilot]

This doc implies a root layout Error Boundary wrapping will catch errors from "any loader in the route tree", but it will not catch errors from the root layout route's own loader (because that boundary renders only if RootLayout renders). Consider clarifying that it catches descendant route loader errors, and mention that to catch the root route loader error you need an Error Boundary above (or avoid a loader on the root layout).

Suggested change

	<code>{"<Outlet />"}</code>. This catches errors from any loader in
	the route tree while keeping the root layout (header, navigation,
	etc.) intact:
	<code>{"<Outlet />"}</code>. This lets you catch errors from
	descendant route loaders while keeping the root layout (header,
	navigation, etc.) intact. To catch errors from the root layout
	route&rsquo;s own loader, place an error boundary above{" "}
	<code>{"<Router />"}</code> (or avoid using a loader on the root
	layout route):