perf(server): skip duplicate App RSC vary merge

[NathanDrake2406]

Overview

This PR avoids running the generic Vary merge path when App Router response finalization sees the internal RSC Vary header is already present, or when the header is missing and can be set directly.

Area	Change	Impact
App Router SSR	Fast-path App RSC Vary finalization	Removes repeated split, trim, dedupe work from the common page response path
Custom Vary responses	Keep using mergeVaryHeader	Preserves existing wildcard and dedupe semantics

Why

App page helpers already stamp VINEXT_RSC_VARY_HEADER before finalizeAppRscResponse() runs. The finalizer was still calling the generic merge helper, which reparsed and deduped the same known header value on every non-static App Router response.

Validation

• vp check tests/app-rsc-response-finalizer.test.ts packages/vinext/src/server/app-rsc-response-finalizer.ts
• vp test run tests/app-rsc-response-finalizer.test.ts
• vp run vinext#build

Perf note

Local 20k warmed SSR profile against the benchmark app showed mergeVaryHeader self-time on this path drop from 70.33ms to 0ms, with finalizeAppRscResponse self-time dropping from 12.29ms to 5.37ms.

Non-goals

This does not change the broader mergeVaryHeader helper or optimize route tree construction. Those are separate hot paths.

[pkg-pr-new]

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commit: f2600d9

[github-actions]

Performance benchmarks

Compared f2600d9 against base 388b5c3 using alternating same-runner rounds. Next.js was unchanged and skipped.

1 improved · 0 regressed · 4 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	129.9 KB	129.9 KB	⚫ 0.0%
Dev server cold start	vinext	2.50 s	2.48 s	⚫ -0.9%
Production build time	vinext	2.80 s	2.75 s	🟢 -1.6%
RSC entry size (gzip)	vinext	64.1 KB	64.1 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	181.7 KB	181.8 KB	⚫ +0.0%

View detailed results and traces

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