Add src-driven frame self-loading and server-scoped frame render

[vivek7405]

Problem

has no src/loading attribute (webjs-frame.js:53-59), so a region cannot self-load its content eagerly on connect or lazily on viewport entry. webjs has lazy COMPONENT module loading (static lazy=true, lazy-loader.js) but no lazy CONTENT loading for a region, forcing the fetch-in-handler anti-pattern AGENTS.md warns against for deferred content (comments, recommendations rail, expensive card). Compounding this, router-client.js:1192 sends x-webjs-frame on every frame fetch but no server file reads it: the server renders the full page (ssr.js:247-262) and the client discards all but the matched frame (router-client.js:1462-1469), so a frame pays full-page render+transfer cost for a region swap, the opposite of the X-Webjs-Have optimization webjs already ships.

Design / approach

Add native src/loading attributes to that fetch on connect or via the existing IntersectionObserver, and read the already-sent x-webjs-frame header in ssr.js to render only the requested subtree when isolable (full-page fallback). Reuses the native observer and the existing X-Webjs-Have-style header short-circuit.

Web-standards fit: Self-loading rides the native IntersectionObserver and a standard request header; the framework adds attribute handling and a render-scope branch, no new loader.

Prior art: Turbo frame_element.js observedAttributes src/loading, FrameLoadingStyle; frame_controller.js appearanceObserver vs #loadSourceURL.

Acceptance criteria

• fetches its content on connect (eager) or on viewport entry (lazy)
• The lazy path reuses the existing IntersectionObserver infra and the existing frame swap path
• A documented server-rendered-fallback caveat covers the JS-disabled baseline
• Browser/e2e tests cover eager self-load and viewport-triggered lazy load
• ssr.js reads x-webjs-frame and renders only the requested frame subtree for an isolable route (full-page fallback otherwise), byte-equivalent to the extracted frame, with a test asserting reduced render for an isolable route
• Tests cover the new behaviour at the applicable layer(s)
• Docs / AGENTS.md / CONVENTIONS.md updated if the public surface changed

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_{Filed from the production-readiness audit (webjs vs Next.js / Remix / Rails / Turbo / Lit). Theme: frames. Priority: P2. Kept to webjs identity: no-build, progressive enhancement, web-components-first, AI-first, batteries-included, close to web standards.}