Build a general-purpose test fixture on UmBootstrap

[deanleigh]

Build a genuine, general-purpose test fixture for UpDoc, in conjunction with UmBootstrap rather than derived from a client site.

Not urgent. Recorded so it is not lost. See planning/TEST_SITE_STRATEGY.md.

Why this is needed eventually

The current UpDoc.TestSite is a partial mirror of Tailored Travel, and #117 makes that leaning explicit by bringing in a full local copy of the client site.

That works while Dean is the only person on UpDoc. It stops working the moment anyone else wants to contribute: nobody outside should need access to a client site to work on an open-source package.

What it needs to be

• Inert and unopinionated. No client-specific document types, no bespoke helpers, no complex layout system
• Self-contained. Its own test PDFs, its own sample content, no external dependencies
• Clonable. Small enough to live in git properly, including its content
• Built on UmBootstrap, which is already a starter template and already Dean's, rather than another bespoke site

What it is for

• Contributors who need a working UpDoc install without client access
• E2E tests that assert on fixtures they control
• Demonstrating UpDoc in docs and recordings without exposing client content

What it is not for

Proving UpDoc works against Tailored Travel. That is what the copy in #117 does. A deliberately simple fixture cannot catch client-specific breakage, and should not try.

Open questions

• Does it replace UpDoc.TestSite, or is it a third host site?
• How much sample content is enough to exercise PDF, web and markdown sources?
• Can the existing E2E suite point at it, or do the specs need rewriting to create their own fixtures? (smoke-test-pdf.spec.ts already creates and cleans up after itself, which is the pattern to follow.)
• Does UmBootstrap's own per-site cookie config (see Give the test site unique backoffice cookie names for local multi-instance development #107) need settling first?