feat(router,website): base-path support for subpath deployments

[stefvw93]

Problem

The GitHub Pages deploy works, but the site is broken at stefvw93.github.io/weft/: every asset and internal link is root-absolute (/assets/…, /docs/…), so JS/CSS fetch from stefvw93.github.io/assets/… → 404, and hard links escape the subpath.

Approach

Route patterns keep their literal strings (segment-string type inference stays intact); the base is handled at the URL boundaries instead.

@weftui/router — new base option (spec: packages/router/src/base.specs.md):

• RouterLive({ base }): strips the base on location reads (seed, popstate), prefixes it on History writes, and the link interceptor matches/navigates on the stripped href — same-origin hrefs outside the base fall through to the browser.
• RouterServer.render({ url, base }): strips before matching; URLs outside the base render the 404 page.
• Canonical invariant: match.url, navigate targets, and everything stored in the router stay base-less. No behavior change when base is omitted.

Website (spec: website/prerender.specs.md, "Subpath deployment"):

• SITE_BASE env → Vite base in both configs; src/lib/site-base.ts exposes the normalized SITE_BASE + withBase().
• All app-rendered internal hrefs go through withBase (docs-shell nav/prev-next/home, landing page, 404 page, markdown links via render-hast).
• Prerender renders SITE_BASE + path but writes base-less output files — the artifact root is the base.
• Build tasks declare env: ["SITE_BASE"]: vp tasks run in a clean environment, so this is required both to pass the variable through and to fingerprint the task cache (a base change was silently reusing stale root-base bundles — including the CI-restored task cache).

CI: the Deploy Website build step sets SITE_BASE=/weft/.

Validation

• vp run check clean; vp run test 597/597 (new: router base helpers + RouterLive/interceptor/server-render base cases, site-base tests); vp run test:browser 36/36.
• E2e: SITE_BASE=/weft/ vp run build:static, served under a /weft/ prefix — all pages 200, entry JS + CSS 200 at /weft/assets/…, zero stray non-base hrefs, 404.html correct.
• Regression: build without SITE_BASE is root-absolute as before (cache correctly invalidates both directions).

After merge: release → deploy runs with the base baked in; site at stefvw93.github.io/weft/ becomes fully functional. If a custom domain is added later, deleting the SITE_BASE env line reverts to root serving.

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