feat(draw): compare template + aspect-fit image painting (T-319)

compareTemplateHandler lowers an `images` array to an SVG of side-by-side
<image> cells, each with the per-object data-label/description caption and
data-lightbox; paths resolve to absolute up front (injected, honest error
on a miss). _paintImage now aspect-fits (contain, centered) so cells of
differing-shape images don't distort. Flutter-free handler + painter pixel
tests. Card image-loading + registration next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/draw/compare_template.dart

@@ -0,0 +1,57 @@
+/// The `compare` drawing-card template (T-319 / D-91 / D-103).
+///
+/// A before/after (or N-up) comparison: the doc's `images` array — each
+/// `{path, label, description}` — lowers to an SVG of side-by-side `<image>`
+/// cells, every cell carrying the per-object `data-label` / `data-description`
+/// (the T-318 caption overlay) and `data-lightbox` (tap-to-zoom, shared with the
+/// image template). The same renderer (T-320) paints it; the painter aspect-fits
+/// each image into its cell so differing shapes don't distort.
+///
+/// Paths are resolved to absolute up front via an injected resolver (like
+/// image.show), so the card loader just reads the file — and an unresolvable
+/// path is an honest [DrawErr], not a broken cell.
+///
+/// Flutter-free: pure Dart, runs under `dart test`.
+library;
+
+import 'draw_dispatch.dart';
+
+/// Resolves a user-supplied image path to an absolute existing path, or null.
+typedef ComparePathResolver = String? Function(String path);
+
+const _cellW = 320, _cellH = 240, _gap = 20, _capH = 56;
+
+/// Handler for `template: "compare"` — reads the doc's `images` array. Register
+/// this in the [DrawingRegistry] with a path [resolvePath] (wired to the
+/// workspace root in main.dart).
+DrawingTemplateHandler compareTemplateHandler({required ComparePathResolver resolvePath}) {
+  return (doc) async {
+    final images = doc.fields['images'];
+    if (images is! List || images.isEmpty) {
+      return const DrawErr('the compare template needs a non-empty "images" array of {path,label,description}');
+    }
+    final cells = StringBuffer();
+    for (var i = 0; i < images.length; i++) {
+      final item = images[i];
+      if (item is! Map) return const DrawErr('each compare image must be an object with a "path"');
+      final path = _str(item['path']);
+      if (path == null) return const DrawErr('each compare image needs a "path"');
+      final abs = resolvePath(path);
+      if (abs == null) return DrawErr('no such image: $path');
+      final x = i * (_cellW + _gap);
+      final label = _str(item['label']);
+      final desc = _str(item['description']);
+      cells.write('<image href="${_esc(abs)}" x="$x" y="0" width="$_cellW" height="$_cellH"');
+      if (label != null) cells.write(' data-label="${_esc(label)}"');
+      if (desc != null) cells.write(' data-description="${_esc(desc)}"');
+      cells.write(' data-lightbox=""/>');
+    }
+    final n = images.length;
+    final totalW = n * _cellW + (n - 1) * _gap;
+    return DrawOk('<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 $totalW ${_cellH + _capH}">$cells</svg>');
+  };
+}
+
+String? _str(Object? v) => v is String && v.trim().isNotEmpty ? v.trim() : null;
+
+String _esc(String s) => s.replaceAll('&', '&amp;').replaceAll('<', '&lt;').replaceAll('>', '&gt;').replaceAll('"', '&quot;').replaceAll("'", '&apos;');