Show a picture whose address was typed without a scheme

`preview.redd.it/lz4drsqh0clh1.jpeg?width=1290&...` was rendering as plain
text -- not a picture, and not even a link. The bare-address rule has always
insisted on `https://`, so anything copied out of an address bar, where the
browser hides the scheme, fell through to nothing at all. That predates the
inline images from the last commit; it just did not matter until pictures
started being worth placing.

The rule is deliberately narrow: a host with a dot, a path, and an image
extension. Widening it to every schemeless address would be the obvious move
and is wrong, because a comment thread is full of dotted, slashed prose --
`src/render/post.ts` would become a link to a website in Tonga, and
`node_modules/foo/bar.js` a website in Jersey. Requiring the extension costs
nothing here, since the only thing worth guessing a scheme for is a picture.

https is assumed. Every host that serves these redirects to it anyway.

Checked both ways: the four address shapes that should become pictures do,
and nine pieces of ordinary prose that must not -- file paths, a Windows
drive letter, a relative path, an email address followed by a filename, a
version number -- still do not.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017nMQ2eDKnqALYhAibpTKTu

test/markdown.test.ts

@@ -122,3 +122,34 @@ test("an image's alt text is escaped like anything else a stranger wrote", () =>
 test('images inside a quote are still placed', () => {
   assert.match(mdi('> ![](https://i.redd.it/x.gif)'), /<blockquote><p><img/);
 });
+
+test('an image address typed without a scheme is still the picture', () => {
+  // Which is how people type them: no https, straight from the address bar.
+  const out = mdi('preview.redd.it/lz4drsqh0clh1.jpeg?width=1290&s=b27e');
+  assert.ok(out.includes('<img src="https://preview.redd.it/lz4drsqh0clh1.jpeg?width=1290&amp;s=b27e"'), out);
+});
+
+test('a schemeless image address mid-sentence keeps the sentence', () => {
+  const out = mdi('look at i.redd.it/x.png nice one');
+  assert.ok(out.startsWith('<p>look at <img'), out);
+  assert.ok(out.endsWith(' nice one</p>'), out);
+});
+
+test('prose full of dots and slashes is not mistaken for an address', () => {
+  // The reason this rule insists on a host, a path and an image extension.
+  for (const text of [
+    'the file is at src/render/post.ts',
+    'see node_modules/foo/bar.js',
+    'a path like ./images/cat.jpg',
+    'C:/Users/x/cat.png',
+    'email [email protected]/nope.jpg',
+    'version 1.2.3/4.png',
+  ]) {
+    assert.ok(!mdi(text).includes('<img'), `treated as an image: ${text}`);
+  }
+});
+
+test('a schemeless address that is not an image is left alone', () => {
+  // Guessing a scheme is worth it for a picture and not for prose.
+  assert.equal(mdi('example.com/article'), '<p>example.com/article</p>');
+});