Fix #194 r2: align line-number gutter in both wrap modes

QA FAIL of PR #179 (verified on palette-dev 2026-09-10) found two defects:

1) Row accounting: splitLines() joined .codeline blocks with '\n' text
   nodes. Since dee062d made .codeline display:block, each interleaved
   newline text node rendered as its own extra visual row in the code
   column under pre-wrap, but renumber() counted only .codeline span
   heights, so the gutter had one fewer row per line break and numbers
   drifted off alignment on wrapped lines. Fix: join('') so blocks are
   adjacent with no phantom newline rows; blank source lines become
   empty blocks given a line box via .codeline:empty::before (ZWJ) in
   app.css so they still count as one row.

2) Inline gutter spans: .gutline spans were appended with no separators
   and .code .gutter .gutline { display: block } did not exist on the
   deployed build, so all numbers landed on one row. That rule is
   present in origin/dev (merged via #183); kept unchanged and verified
   by rendering in a local build.

Wrap OFF path unchanged: gutter still gets '1\n2\n3\n4' text and the
horizontal-scroll gutter behavior is untouched (no CSS changes to
scrolling; only a :empty line-box rule). CSP unchanged: no inline
styles/handlers, static JS only.

internal/web/static/paste-lines.js

@@ -13,7 +13,13 @@
   };
 
   // Wrap each logical line (split on newline; spans never contain newlines
-  // because HighlightCode highlights per line) in a .codeline block.
+  // because HighlightCode highlights per line) in a .codeline block. The
+  // blocks must be adjacent with NO newline text between them (#194):
+  // .codeline is display:block, and under pre-wrap each interleaved '\n'
+  // text node renders as an extra phantom row in the code column that
+  // renumber() does not count, drifting the gutter off alignment on
+  // wrapped lines. Blank lines become empty blocks; .codeline:empty keeps
+  // them one row tall (see app.css).
   function splitLines() {
     var html = body.innerHTML;
     var parts = html.split('\n');
@@ -21,7 +27,7 @@
     for (var i = 0; i < parts.length; i++) {
       out.push('<span class="codeline">' + parts[i] + '</span>');
     }
-    body.innerHTML = out.join('\n');
+    body.innerHTML = out.join('');
   }
 
   function renumber() {