track the demo as a real site in examples/, gated like the docs

The human asked for a demo extensive enough to review by hand, tracked as files,
and kept current the way docs are. Generated filler cannot be reviewed — you cannot
read a Go function and see what a reader sees — so the generator is deleted and
`examples/demo-site/` is an ordinary site: 33 Markdown files, six pictures,
site.yaml, a template override and static files.

My reason for generating it was ADR-0011, and I had that rule wrong: it puts the
*author's site root* outside this repository, not fixtures. conventions.md already
keeps golden files in testdata/, and this is the same category one size up. Keeping
both a generator and files would have been two sources of truth, so one had to go.

Kept true by two gates rather than by good intentions. A table-driven test in
internal/web serves the directory through the real handler with one case per
feature — thirty-five of them, each naming what it proves — and verify.sh runs
`khosra check` over it and fails on anything fatal. Adding a feature now means
adding it to the demo and adding its case, and the build says so if you don't.

Extensive on purpose: eleven dated posts so a section actually paginates, a
four-chapter series so first/last are not the neighbours, a gallery with three
JPEGs and an SVG so both the resampled and the untouched paths show, a Bengali-only
bundle, a titleless status, a draft, a future date, an include, a nested extras
tree, and a typography page that demonstrates what the engine will not do.

Two expectations of mine were wrong and the demo corrected them: the site's own
list template overrides *tag* listings too, so there are no group headings there —
which turns out to be the better demonstration of ADR-0046, since the theme picking
the flat shape is exactly the point. And template literal text is not escaped, so
an apostrophe stays an apostrophe.

examples/demo-site/content/writing/typography.en.md

@@ -0,0 +1,17 @@
+---
+title: Typography and Its Limits
+date: 2026-03-28
+tags: [journal]
+---
+"Quotes become curly," she said -- and dashes become dashes, while an ellipsis... becomes one character. That
+is the whole of what the engine does to an author's words: a Markdown parser option, nothing more.
+
+Inside a code span none of it happens: `"quotes" -- and ellipses...` survive byte for byte, because the
+typographer works on the parsed tree rather than on the text.
+
+Line breaking is not the engine's business at all. This paragraph is deliberately long enough to wrap several
+times on a narrow screen, which is where you would notice a lonely last word — and the stylesheet asks the
+browser to avoid one with `text-wrap: pretty`, because only the browser knows where the lines actually fall.
+
+    A code block, indented.
+    Also untouched.