Add a trailing-slash policy that canonicalizes with a redirect

[vivek7405]

Problem

Because route patterns end in /?$ (router.js buildRoutePattern:237), /about and /about/ both serve a 200 with identical content and there is no canonical redirect to one form. This is duplicate-content: search engines index both URLs, link-equity splits, analytics double-counts, and CDN cache keys split on the slash, halving hit rate. There is no config to pick a policy and no redirect to enforce it.

Design / approach

In handle(), before matchPage, normalize the native URL pathname per a webjs.trailingSlash setting and return a literal 308 to the canonical form (never the root). Pure URL string handling and a standard redirect.

Web-standards fit: Operates on the native URL object and emits a standard 308; the policy is a tiny string check, no router rewrite engine.

Prior art: Next.js trailingSlash:false defaults and 308-redirects; SvelteKit trailingSlash route option; Rails strips by convention.

Acceptance criteria

• A package.json webjs.trailingSlash setting (default 'never') controls canonicalization
• With 'never', GET /about/ returns 308 to /about (and 'always' does the inverse), preserving query and method semantics
• The root path / is never redirected to empty
• 'ignore' preserves the current both-forms-200 behaviour for back-compat
• Applies in dev.js and the prod handler before SSR and the 404 fallback
• Unit + e2e network-probe: canonical form 200, non-canonical form a single 308 (no loop)
• Tests cover the new behaviour at the applicable layer(s)
• Docs / AGENTS.md / CONVENTIONS.md updated if the public surface changed

---

_{Filed from the production-readiness audit (webjs vs Next.js / Remix / Rails / Turbo / Lit). Theme: routing. Priority: P1. Kept to webjs identity: no-build, progressive enhancement, web-components-first, AI-first, batteries-included, close to web standards.}