fix(cache): honor CDN adapter build identity capability

[james-elicx]

Summary

• emit the HTML buildIdentity warm-plan guarantee only when the configured CDN adapter declares response-header support
• pass the adapter capability through both vinext build discovery and Cloudflare deploy discovery
• keep the independent RSC build identity available for canonical RSC validation
• prevent manifest-driven warming with custom strict-Vary adapters from requiring a header they do not promise

Tests

• vp test run tests/prerender-paths.test.ts tests/cloudflare-cdn-warm-deploy.test.ts tests/deploy-prerender-config.test.ts tests/deploy.test.ts
• vp check packages/vinext/src/build/prerender-paths.ts packages/vinext/src/cli.ts packages/cloudflare/src/deploy.ts tests/prerender-paths.test.ts

Stacked on #3053.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@3054

npm i https://pkg.pr.new/create-vinext-app@3054

npm i https://pkg.pr.new/@vinext/types@3054

npm i https://pkg.pr.new/vinext@3054

commit: 26b3b5e

[github-actions]

Performance benchmarks

Compared 26b3b5e against base a13125b using alternating same-runner rounds. Next.js was unchanged and skipped.

1 improved · 0 regressed · 5 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	142.2 KB	142.2 KB	⚫ -0.0%
Client entry size (gzip)	vinext	129.5 KB	129.5 KB	⚫ -0.0%
Dev server cold start	vinext	3.29 s	3.21 s	🟢 -2.4%
Production build time	vinext	3.71 s	3.70 s	⚫ -0.3%
RSC entry closure size (gzip)	vinext	116.1 KB	116.2 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	197.7 KB	197.7 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
web	preview	production

[james-elicx]

Superseded by consolidated PR #3057. No commits were dropped or rewritten: #3057 now contains this exact linear commit as part of #3047–#3057. Closing only the redundant draft PR.