Add the Developed in dejima badge

The same badge and the same asset as dejima carries, placed where dejima places it: at the end of the opening block, before the first section heading.

test/links.bats only knew about <img src="...png">. The badge is markdown image syntax and an svg, so neither half of that test would have seen it - the assertion would have kept passing while pointing at nothing. It now walks every image the README references in either syntax and either format, and requires at least two so a README that lost one of them fails rather than passing with the other.

Checked by breaking the badge path and watching the guard name it.

README.md

@@ -33,6 +33,8 @@ you what that bound actually is.
 a fix, not a release. [SECURITY.md](SECURITY.md) has the detail, and is where to
 report a vulnerability rather than in a public issue.
 
+[![Developed in dejima](assets/dejima-badge.svg)](https://gitbox.branderson.io/dejima/dejima)
+
 ## What a devbox is, and why it is a different problem
 
 A devbox is a long-lived VM that runs [dejima](https://gitbox.branderson.io/dejima/dejima)

assets/dejima-badge.svg

@@ -0,0 +1,21 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="122" height="20" role="img"
+     aria-label="Developed in dejima">
+  <title>Developed in dejima</title>
+  <linearGradient id="g" x2="0" y2="100%">
+    <stop offset="0" stop-color="#bbb" stop-opacity=".1"/>
+    <stop offset="1" stop-opacity=".1"/>
+  </linearGradient>
+  <clipPath id="r"><rect width="122" height="20" rx="3" fill="#fff"/></clipPath>
+  <g clip-path="url(#r)">
+    <rect width="79" height="20" fill="#282828"/>
+    <rect x="79" width="43" height="20" fill="#8ec07c"/>
+    <rect width="122" height="20" fill="url(#g)"/>
+  </g>
+  <g font-family="Verdana,DejaVu Sans,Geneva,sans-serif" font-size="11"
+     text-anchor="middle">
+    <text x="39.5" y="15" fill="#010101" fill-opacity=".3">Developed in</text>
+    <text x="39.5" y="14" fill="#ebdbb2">Developed in</text>
+    <text x="100.5" y="15" fill="#010101" fill-opacity=".3">dejima</text>
+    <text x="100.5" y="14" fill="#282828">dejima</text>
+  </g>
+</svg>

test/links.bats

@@ -73,12 +73,22 @@ anchors_of() {
     fi
 }
 
-@test "the README's wordmark resolves to a file this repository ships" {
-    src=$(sed -n 's/.*<img src="\([^"]*\)".*/\1/p' "$REPO/README.md" | head -1)
-    [ -n "$src" ]
-    [ -f "$REPO/$src" ]
-    # Both modes ship, so a mirror on a light-themed host changes one line
-    # rather than needing a file it does not have.
+@test "every image the README references is a file this repository ships" {
+    # Both syntaxes and both formats. The wordmark is an <img> tag and a png;
+    # the badge is markdown image syntax and an svg. A test written for only
+    # the first pair would not have seen the second at all.
+    found=0
+    while IFS= read -r src; do
+        [ -n "$src" ] || continue
+        case "$src" in http*) continue ;; esac
+        found=$((found + 1))
+        [ -f "$REPO/$src" ] || { echo "README references missing image: $src"; false; }
+    done < <(grep -ohE 'src="[^"]+\.(png|svg)"|\]\([^)]+\.(png|svg)\)' "$REPO/README.md" \
+             | sed 's/^src="//; s/"$//; s/^](//; s/)$//')
+    [ "$found" -ge 2 ] || { echo "expected at least the wordmark and the badge"; false; }
+
+    # Both wordmark modes ship, so a mirror on a light-themed host changes one
+    # line rather than needing a file it does not have.
     [ -f "$REPO/assets/dejima-wordmark-darkmode.png" ]
     [ -f "$REPO/assets/dejima-wordmark-lightmode.png" ]
 }