parse :name: as an icon, and let the theme decide what one is

The engine's half is one call: parse the name, hand it to a single `icon`
fragment, decide nothing else. No icon table in Go, ever. The surveyed engines
split three ways — Unicode (Hugo, Pandoc), a remote image per icon (Jekyll's
jemoji, which fails the sovereignty test outright), and inlined SVG from a
bundled set (MkDocs Material) — and all three are decisions about markup, which
ADR-0036 puts in the theme. A theme wanting Font Awesome ships a sprite in its
own base.html and redefines one fragment: no webfont, no request, no script.

The boundary rules are the whole difficulty, because the colon is the commonest
punctuation in technical prose. A name must start with a letter, hold only
letters, digits, hyphens and underscores, and neither colon may touch an
alphanumeric. Verified on the real binary that 10:30:15, key:value:pair,
"Note: this", a URL and a code span all come through untouched — each of them
would otherwise be a silent edit to someone's sentence.

The literal fallback closes the same hole from the other side: when the theme
renders nothing, the engine writes the author's `:name:` back, so an
unrecognised icon is never deleted from the middle of a paragraph.

An icon needed its own inline node rather than the block one — goldmark
distinguishes the two by type — which also avoids the type switch CLAUDE.md §6
forbids: two kinds, two renderer functions.

The reference theme maps six names to Unicode and ships no sprite, font or
asset. core 2794/2800, ext 1615/2000, 34 gates green.

examples/demo-site/content/writing/notes-on-water/index.en.md

@@ -23,6 +23,9 @@ dropped. Readings go to the NIWA archive, and the definition below expands every
 
 *[NIWA]: National Institute of Water and Atmospheric Research
 
+:warn: The south gauge reads low after a storm — the theme decides what that icon is, and an unknown one like
+:nosuchicon: keeps its text. Times like 10:30:15 and pairs like key:value:pair are left alone.
+
 | Gauge | Reading | Note |
 |-------|---------|------|
 | North | 41 mm | steady |