feat(core): support formaction=${action} on submitters (#1207)

[vivek7405]

Closes #1207

Overview

This PR adds support for formaction=${action} on submitters (<button> and <input type="submit">) inside bound forms (<form action=${formAction}>).

Key Changes

1. Core Runtime (form-action.js, render-server.js, render-client.js):

   • Recognized formaction=${action} on <button> and <input> submitters.
   • Refused unparseable options: submitter name attribute, input type="image", formmethod="get", formenctype="text/plain", or submitter outside a bound <form action=${...}>.
   • In SSR (renderTemplate & streamTemplate), deferred submitter validation to tag-end close (>) to inspect author-written attributes following formaction=${action}.
   • Emits <button name="__webjs_action" value="<hash>/<fn>"> directly into rendered HTML.
   • Updated client-side reconciliation (buildFormActionRecord, reconcileFormActions, reconcileSubmitterAction) for CSR.

2. Server Action Dispatch (form-dispatch.js):

   • Extracted formData.getAll(FORM_ACTION_FIELD) and selected actions[actions.length - 1], matching standard WHATWG DOM order last-wins submitter precedence with JS OFF.

3. Auditor (check.js):

   • Extended form-action-not-a-get-action to check submitters (<button> and <input>).

4. Testing & Docs:

   • Added unit tests across core (form-action-binding.test.js, form-action-binding-client.test.js, form-action-attr-guard.test.js, form-action-attr-guard-client.test.js), server (form-dispatch.test.js), and Bun parity (test/bun/form-action-submitter-parity.test.mjs, test/bun/form-action-guard.mjs).
   • Updated documentation in AGENTS.md, .agents/skills/webjs/references/, and website/app/docs/{server-actions,progressive-enhancement,migrating-from-nextjs,troubleshooting}/page.ts.