refactor(app): define response-stage protocol

[james-elicx]

Part 7 of 16 in the staged CDN adapter stack. Depends on #3146.

Summary

• defines the App Router response-stage protocol and serializable element wire format
• keeps App-specific request props and validation out of the generic stage contracts
• covers protocol serialization, malformed input, and stage dispatch behavior

Validation

• focused App element-wire and worker-stage tests
• cumulative exact-tip changed-test sweep: 3,461 passed, 2 skipped
• full exact-tip vp check

Review size

• Implementation: +330 / -76 LOC
• Tests and fixtures: +83 / -0 LOC
• Other (docs, config, lockfile): +0 / -0 LOC
• 5 changed files

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

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@3147

npm i https://pkg.pr.new/create-vinext-app@3147

npm i https://pkg.pr.new/@vinext/types@3147

npm i https://pkg.pr.new/vinext@3147

commit: 14d0de0

[github-actions]

Performance benchmarks

Compared 14d0de0 against base 2548828 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	3.06 s	3.04 s	⚫ -0.8%
Production build time	vinext	3.32 s	3.32 s	⚫ -0.1%
RSC entry closure size (gzip)	vinext	119.8 KB	119.8 KB	⚫ -0.0%
Server bundle size (gzip)	vinext	203.5 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