fix(macos): scope App Transport Security exception

[SarthakWade]

Summary

• remove the process-wide NSAllowsArbitraryLoads exception from the macOS app
• retain HTTP compatibility only inside WKWebView with NSAllowsArbitraryLoadsInWebContent
• add a packaging invariant that validates the plist and rejects any future blanket ATS exception
• document the scoped macOS HTTP boundary and mark backlog item E8 complete

Security rationale

Apple documents NSAllowsArbitraryLoadsInWebContent as the scoped way to allow web-view HTTP while keeping ATS enabled for native application networking. This preserves Headless's HTTP/HTTPS browser contract without weakening unrelated app connections.

Reference: https://developer.apple.com/documentation/bundleresources/information-property-list/nsapptransportsecurity/nsallowsarbitraryloadsinwebcontent

Verification

• macOS production app build and positive plist invariant
• negative plist mutation proving NSAllowsArbitraryLoads is rejected
• pnpm test (44/44 protocol tests and MCP integration)
• pnpm test:runtime
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• Prettier check on all changed Markdown files
• pnpm test:e2e:mac over the local HTTP fixture
• pnpm test:e2e:linux

Closes #46