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/notes-on-water/extras/gauge.log

@@ -0,0 +1,3 @@
+day one: 2m, rising
+day two: 3m, holding <not markup>
+day three: 3m, falling