App Router: misc navigation regressions (unicode params, useParams identity, push pending)

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

A handful of app-router navigation regressions: (1) unicode search params not surfaced in the RSC ok-response stream; (2) useParams returns 0 items (no identity stability) when rendered under pages; (3) external router.push() reports lastIsPending: false instead of true. These do not cleanly fit any single existing issue.

unicode search params: expected pathname "/" with content-type text/x-component

useParams identity: Expected length 2, Received length 0

Estimated Impact

~4 test failures across the deploy suite.

Affected Test Suites

• test/e2e/app-dir/navigation/navigation.test.ts (3 failures)
• test/e2e/app-dir/search-params-react-key/layout-params.test.ts

Recommendation

1. Reproduce first in vinext's own test suite. Cover each regression independently: unicode search params in RSC stream, useParams identity stability, external router.push pending status.

2. Address each root cause. Unicode: ensure search params are decoded/encoded consistently on the SSR path. useParams: stabilise the identity across renders. External push: thread the pending status back to the consumer.

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Part of #1328.