add inline notation, and take the tilde back from strikethrough

~sub~, ^sup^, ==mark==, and ~~strike~~ moved in from goldmark. Not a
preference: goldmark's strikethrough claims a single tilde as well as a double,
so with it enabled H~2~O rendered as H<del>2</del>O — measured before the
change. Two features cannot share a byte and both be correct, so notation owns
it and the authored syntax stays exactly as ADR-0058 documented.

The second failure was worse and only showed up under test. Under delimiter
rules `x^2 + y^2 = z^2` pairs its carets across the whole expression and renders
x<sup>2 + y</sup>2 — prose silently becoming markup, in exactly the content this
engine is for. So a single run is scanned rather than paired, and may not cross
whitespace: a subscript holds a formula, never a phrase. Pandoc draws the same
line. The cost is that a single run takes its content literally, so there is no
emphasis inside a subscript, which the ADR states rather than leaving to be
discovered.

New package under internal/ext, which is a stop condition and was asked. It
takes the extensions counter to 4, past its threshold, and the answer is still
no: four features attach in three unrelated ways, and two goldmark extenders
compose in goldmark's own extender list, which is already the registry for that
shape.

The example site's hand-copied extender list drifted, exactly as the latent row
added last loop predicted — the demo case failed and named it. Both are now in
step again.

core 2794/2800, ext 1236/2000, 34 gates green, 0 warnings.

internal/web/example_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/yuin/goldmark/extension"
 
 	"khosra/internal/content"
+	"khosra/internal/ext/notation"
 	"khosra/internal/ext/shortcodes"
 	"khosra/internal/render"
 )
@@ -49,7 +50,7 @@ func exampleSite(t *testing.T) http.Handler {
 			extension.Table,
 			extension.NewFootnote(extension.WithFootnoteIDPrefixFunction(shortcodes.FootnotePrefix)),
 			extension.DefinitionList,
-			extension.Strikethrough,
+			notation.New(),
 			shortcodes.New(p),
 		}
 	})
@@ -119,7 +120,10 @@ var exampleFeatures = []featureCase{
 		expect: []string{`<h2 id="method">Method</h2>`, "<em>Emphasis and links survive</em>"}},
 	{what: "a fragment is not a bundle", path: "/writing/notes-on-water/_method/", code: 404},
 	{what: "authored HTML renders, because the site root is trusted", path: "/writing/notes-on-water/", code: 200,
-		expect: []string{"H<sub>2</sub>O", "<kbd>Shift</kbd>"}, absent: []string{"raw HTML omitted"}},
+		expect: []string{"<kbd>Shift</kbd>"}, absent: []string{"raw HTML omitted"}},
+	{what: "notation marks become elements, and a single tilde is a subscript not a strike", path: "/writing/notes-on-water/", code: 200,
+		expect: []string{"H<sub>2</sub>O", "10<sup>-3</sup>", "<mark>important</mark>", "<del>a struck phrase</del>"},
+		absent: []string{"<del>2</del>"}},
 	{what: "the dialect renders tables, definition lists and strikethrough", path: "/writing/notes-on-water/", code: 200,
 		expect: []string{"<table>", "<th>Gauge</th>", "<dl>", "<dt>Monsoon</dt>", "<del>a struck phrase</del>"}},
 	{what: "a fragment's footnote ids are namespaced, so the page's own keep working", path: "/writing/notes-on-water/", code: 200,