fix(server): sever RevealGroupContext in createErrorBoundary to match client behavior

[AmariahAK]

Description

## Summary

The server-side `createErrorBoundary` did not sever `RevealGroupContext` for its
children, unlike the client-side `createCollectionBoundary` which clears the
reveal controller context for all boundary types (both Loading and Errored).

This caused an `<Errored>`-wrapped `<Loading>` to incorrectly register with an
ancestor `<Reveal>` group on the server, holding the entire group to the slowest
slot on hard refresh while SPA navigation revealed independently.

**Fix:** Moved `RevealGroupContext` from `server/hydration.ts` to
`server/signals.ts` (resolving a circular dependency), then added
`setContext(RevealGroupContext, null, owner)` in `createErrorBoundary` — matching
the client's pattern at `boundaries.ts:386`.

Closes #2872.

## How did you test this change?

```bash
# Server-side reveal tests (27 passed, including the new test)
pnpm build && cd packages/solid && npx vitest run test/server/reveal-ssr.spec.ts

# Client-side reveal order tests (29 passed — no regressions)
cd packages/solid-signals && npx vitest run tests/createRevealOrder.test.ts

# Full server test suite (173 passed)
cd packages/solid && npx vitest run test/server/

New test added: "Errored-wrapped Loading does not join ancestor Reveal group" in
reveal-ssr.spec.ts — verifies that only the direct Loading registers in the
group and the Errored-wrapped one does not.

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Latest commit: 7bffb17

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