refactor(cache): isolate response-stage admission

[james-elicx]

Part 4 of 16 in the staged CDN adapter stack. Depends on #3143.

Summary

• extracts response-stage cacheability policy and clean-EOF admission into typed server helpers
• carries only trusted prerender and revalidation state across a stage boundary
• preserves draft-mode, middleware, header, cookie, static-file, and route-policy exclusions
• keeps generated entries thin and gives the admission lifecycle direct regression coverage

Validation

• focused cacheability, request-pipeline, cache-control, and shim tests
• full exact-tip vp check
• cumulative exact-tip changed-test sweep: 3,461 passed, 2 skipped

Review size

• Implementation: +640 / -194 LOC
• Tests and fixtures: +842 / -12 LOC
• Other (docs, config, lockfile): +0 / -0 LOC
• 28 changed files

Measured against codex/cdn-v2-adapter-runtime after rebasing onto main@a6931c0.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@3144

npm i https://pkg.pr.new/create-vinext-app@3144

npm i https://pkg.pr.new/@vinext/types@3144

npm i https://pkg.pr.new/vinext@3144

commit: d05a2c4

[github-actions]

Performance benchmarks

Compared d05a2c4 against base 030335e using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 1 regressed · 5 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	142.3 KB	142.3 KB	⚫ +0.0%
Client entry size (gzip)	vinext	129.6 KB	129.6 KB	⚫ +0.0%
Dev server cold start	vinext	3.32 s	3.39 s	🔴 +2.2%
Production build time	vinext	3.52 s	3.53 s	⚫ +0.1%
RSC entry closure size (gzip)	vinext	119.7 KB	119.8 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	203.4 KB	203.4 KB	⚫ +0.0%

View detailed results and traces

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

[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
static-export	preview	production
web	preview	production