refactor(pages): extract response stage

[james-elicx]

Part 5 of 16 in the staged CDN adapter stack. Depends on #3144.

Summary

• extracts the Pages response renderer into an importable typed stage
• keeps API routes, page rendering, preview state, headers, and response policy in Pages-owned modules
• defines the Pages stage wire contract without selecting a transport
• bypasses shared response dispatch for tokenized WebSocket upgrades

Validation

• focused Pages API, page-handler, response-stage, route-data, and worker-stage tests
• cumulative exact-tip changed-test sweep: 3,461 passed, 2 skipped
• full vp check and vinext package build

Review size

• Implementation: +609 / -48 LOC
• Tests and fixtures: +1166 / -3 LOC
• Other (docs, config, lockfile): +0 / -0 LOC
• 15 changed files

Measured against codex/cdn-v2-stage-cacheability after rebasing onto main@a6931c0.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@3145

npm i https://pkg.pr.new/create-vinext-app@3145

npm i https://pkg.pr.new/@vinext/types@3145

npm i https://pkg.pr.new/vinext@3145

commit: 0acbae4

[github-actions]

Performance benchmarks

Compared 0acbae4 against base d05a2c4 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	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	3.11 s	3.11 s	⚫ +0.1%
Production build time	vinext	3.32 s	3.31 s	⚫ -0.3%
RSC entry closure size (gzip)	vinext	119.8 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