feat(build): select adapter-owned worker stages

[james-elicx]

Part 10 of 16 in the staged CDN adapter stack. Depends on #3149.

Summary

• atomically publishes adapter-selected staged-output descriptors and lazy request/response loader exports
• resolves and emits separate request and response stage entries during production builds
• preserves the existing single-entry fetch handler when no staged output is selected
• keeps pregenerated-route lookup available to selected entries and excludes client-only environments

Validation

• focused build-optimization, entry-template, fetch-handler, and adapter-config tests
• 188 publication/build-boundary assertions passed, 2 platform skips
• cumulative exact-tip changed-test sweep: 3,461 passed, 2 skipped

Review size

• Implementation: +265 / -22 LOC
• Tests and fixtures: +550 / -12 LOC
• Other (docs, config, lockfile): +8 / -0 LOC
• 14 changed files

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

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@3150

npm i https://pkg.pr.new/create-vinext-app@3150

npm i https://pkg.pr.new/@vinext/types@3150

npm i https://pkg.pr.new/vinext@3150

commit: 01bb5ba

[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

[github-actions]

Performance benchmarks

Compared 01bb5ba against base daa9683 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	3.37 s	3.38 s	⚫ +0.3%
Production build time	vinext	3.72 s	3.77 s	⚫ +1.3%
RSC entry closure size (gzip)	vinext	125.6 KB	125.6 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	216.4 KB	216.4 KB	⚫ +0.0%

View detailed results and traces

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