ship the side art as a cached stylesheet so cached loads paint in one frame

css/style.css

@@ -189,8 +189,10 @@ button {
 
 /* ── side art ───────────────────────────────────────────────────────────
    Full-height sakura in both margins: ANSI art (inc/art/sakura.ans, `cat`
-   it), turned into HTML by inc/art.php and mirrored for the right side so the
-   branches hang in from the outer corners. Hidden when there's no margin. */
+   it), turned by inc/art.php into one text layer per color, served as
+   sideart.php's ::before content on the empty <b>s and mirrored for the right
+   side so the branches hang in from the outer corners. Hidden when there's
+   no margin. */
 
 /* The art is drawn for .6em-wide characters; system monospaces range from
    .5em up, which squashes it. So it gets its own: JetBrains Mono's printable
@@ -217,7 +219,7 @@ button {
 }
 /* the art comes as one layer per color, stacked */
 .side { display: grid; }
-.side > span { grid-area: 1 / 1; direction: ltr; unicode-bidi: isolate; }
+.side > b { grid-area: 1 / 1; font-weight: normal; direction: ltr; unicode-bidi: isolate; }
 .side-left { left: 0; }
 /* rtl only so the right panel clips on its inner edge; the layers stay ltr
    (above), or the bidi algorithm reorders runs of punctuation */