fix(link): keep auto prefetch from fetching dynamic app routes

[NathanDrake2406]

Overview

Field	Detail
Goal	Stop App Router <Link prefetch={null}>, omitted prefetch, and prefetch=\"auto\" from behaving like prefetch={true} for dynamic routes.
Core change	Split Link prefetch into disabled, auto, and full, then gate automatic full RSC prefetches through a generated static route manifest.
Main boundary	prefetch={true} still opts into full RSC prefetch. null/undefined/\"auto\" no longer trigger full dynamic route payload fetches.
Primary files	packages/vinext/src/shims/link.tsx, packages/vinext/src/entries/app-browser-entry.ts, packages/vinext/src/index.ts
Expected impact	Less background work for dynamic App Router links that use default/auto prefetch semantics.

Why

The Link prefetch prop is not a boolean in the App Router contract. Next.js treats null, undefined, and \"auto\" as automatic prefetch, while true is the explicit full-prefetch mode. Vinext collapsed those states into “enabled”, which meant default links to dynamic routes could fetch full RSC payloads as soon as they entered the viewport.

Area	Principle / invariant	What this PR changes
Link API	Public prop values must preserve the Next.js distinction between auto and full.	Adds a typed prefetch mode resolver instead of using prefetch !== false.
App Router RSC prefetch	Automatic full prefetch should only happen when vinext can prove it is a static route shape.	Emits a small browser route manifest and matches hrefs against it before full auto prefetch.
Explicit opt-in	prefetch={true} is still allowed to fetch the full route and data.	Keeps explicit full prefetch behavior unchanged.

What changed

Scenario	Before	After
<Link href=\"/about\" /> for a static app route	Full RSC prefetch	Full RSC prefetch
<Link href=\"/blog/hello-world\" /> for a dynamic app route	Full RSC prefetch	No full auto RSC prefetch
<Link href=\"/blog/hello-world\" prefetch={true} />	Full RSC prefetch	Full RSC prefetch
Dangerous URL schemes	Disabled	Disabled

Maintainer review path

1. packages/vinext/src/shims/link.tsx: mode resolution, static route matching, and the auto-prefetch gate.
2. packages/vinext/src/entries/app-browser-entry.ts: browser route manifest generation.
3. packages/vinext/src/index.ts: passes scanned App Router routes into the browser entry generator.
4. tests/link.test.ts and tests/entry-templates.test.ts: focused behavior and codegen coverage.

Validation

• vp test run tests/link.test.ts tests/entry-templates.test.ts
• vp check tests/link.test.ts tests/entry-templates.test.ts packages/vinext/src/shims/link.tsx packages/vinext/src/entries/app-browser-entry.ts packages/vinext/src/client/vinext-next-data.ts packages/vinext/src/index.ts
• git diff --check

Risk / compatibility

• Public API: widens the shim type to accept prefetch=\"auto\" | null, matching App Router Link.
• Runtime: static App Router links still auto-prefetch full RSC payloads. Dynamic App Router links require prefetch={true} for full prefetch.
• Build output: app browser entry now includes a compact route-shape manifest for page routes.
• Intentional scope: Next.js can prefetch loading-boundary shells for some dynamic routes. Vinext currently caches complete RSC responses, not partial segment prefetches, so this PR avoids pretending auto can safely do that work.

Non-goals

• Implement Next.js segment-cache or loading-boundary partial prefetches.
• Include generated static param values in the browser prefetch manifest.
• Change router.prefetch(), which is already an explicit imperative prefetch API.

References

Reference	Why it matters
Next.js App Router Link prefetch prop	Documents \"auto\", null, and undefined as automatic App Router prefetch, distinct from true.
Next.js prefetch mode mapping	Maps true to full prefetch and null/\"auto\" to automatic/PPR behavior.
Next.js PrefetchKind docs	States that auto prefetches static pages fully and dynamic pages partially.
Next.js prefetch=\"auto\" test	Confirms \"auto\" is an alias for the default automatic behavior.

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