refactor(rsc): share canonical prefetch request resolution

[james-elicx]

Summary

• extract the shared canonical RSC request decision used by Link and router.prefetch()
• centralize full/loading-shell header normalization, canonical URL selection, and rewrite alias generation
• preserve contextual identities for rewrites, interceptions, mounted slots, route-tree prefetches, and prefetch inlining
• keep Link and programmatic prefetch scheduling/cache behavior separate

Next.js similarly routes Link and programmatic prefetches through shared segment-cache scheduling primitives:

• https://github.com/vercel/next.js/blob/canary/packages/next/src/client/components/links.ts
• https://github.com/vercel/next.js/blob/canary/packages/next/src/client/components/app-router-instance.ts

Test plan

• vp test run tests/app-prefetch-rsc-request.test.ts tests/prefetch-cache.test.ts tests/link-navigation.test.ts
• vp test run tests/app-rsc-cache-busting.test.ts tests/link.test.ts
• vp check packages/vinext/src/shims/internal/app-prefetch-rsc-request.ts packages/vinext/src/shims/navigation.ts packages/vinext/src/shims/link.tsx tests/app-prefetch-rsc-request.test.ts
• vp run vinext#build

Stacked on #3017.

[pkg-pr-new]

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npm i https://pkg.pr.new/vinext@3019

commit: d7f6655

[github-actions]

Example	Preview	Production	Original
app-router-cloudflare	preview	production
pages-router-cloudflare	preview	production
app-router-playground	preview	production	original
realworld-api-rest	preview	production
nextra-docs-template	preview	production
benchmarks	preview	production
hackernews	preview	production	original
workers-cache	preview	production
web	preview	production

[github-actions]

Performance benchmarks

Compared d7f6655 against base 134a34b using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 0 regressed · 6 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	141.8 KB	142.0 KB	⚫ +0.1%
Client entry size (gzip)	vinext	129.2 KB	129.3 KB	⚫ +0.1%
Dev server cold start	vinext	2.57 s	2.56 s	⚫ -0.4%
Production build time	vinext	2.73 s	2.76 s	⚫ +1.1%
RSC entry closure size (gzip)	vinext	116.0 KB	116.0 KB	⚫ -0.0%
Server bundle size (gzip)	vinext	197.5 KB	197.5 KB	⚫ -0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}