docs: document submit-state events, optimistic(), and view transitions on the client-router page

[vivek7405]

Problem

Two client-router behaviours are documented in agent-docs/advanced.md and AGENTS.md but entirely absent from the docs site:

1. Form submission-state events and the optimistic-UI helper (Add form submission-state events and an optimistic-UI mutation helper #246): webjs:submit-start / webjs:submit-end events, the in-flight form aria-busy, and optimistic(signal, value, action). The docs site client-router page documents only webjs:frame-busy (frame-nav busy state), not form submission state, and the word "optimistic" appears on the site only as an unrelated loading-skeleton mention.
2. View Transitions on partial swaps and permanent-element preservation (View transitions on partial swaps and permanent-element preservation #250): the <meta name="view-transition"> opt-in and data-webjs-permanent. A grep for view.transition|startViewTransition|data-webjs-permanent across docs/app/docs/** returns nothing.

Implementation notes (for the implementing agent)

• Where to edit: docs/app/docs/client-router/page.ts (add a forms/submission-state subsection and a view-transitions subsection).
• What to document for Add form submission-state events and an optimistic-UI mutation helper #246: the webjs:submit-start / webjs:submit-end events with their detail shapes ({form,url} and {form,url,ok}), the in-flight aria-busy on the submitting form, and optimistic(signal, value, action) (set immediately, run the action, roll back on error or { success: false }).
• What to document for View transitions on partial swaps and permanent-element preservation #250: the <meta name="view-transition" content="same-origin"> opt-in (applies to marker, frame, and full-body swaps with a feature-detected fallback) and data-webjs-permanent plus its required id to persist a live element across swaps.
• Source of truth to copy from: agent-docs/advanced.md (around L456 to L504 for submit events / optimistic, around L628 to L694 for view transitions), AGENTS.md around L183 and L306, packages/cli/templates/AGENTS.md around L907 to L916.

Acceptance criteria

• Client-router docs page documents webjs:submit-start / webjs:submit-end, form aria-busy, and optimistic().
• Client-router docs page documents the View Transitions opt-in and data-webjs-permanent.