Add a summary report set from Markdown

from-markdown takes a hand-written summary such as SUMMARY.md and sets it
in the full report's dress: the same cover, header, footers, table of
contents, headings and grids. The shared macros move to macros.html.j2 so
both templates draw from them, and layout.render_pdf takes the template and
document title to use.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

report_generator/template/report.css

@@ -470,3 +470,70 @@ p.note {
 .toc a::after {
     content: leader(".") target-counter(attr(href), page);
 }
+
+/* --- Prose (the summary report) ----------------------------------------------------------- */
+
+/* The summary is written in Markdown, so it brings what the full report never has: running text,
+   lists and links. Set to the full report's measure and rhythm, so the two read as one series. */
+.prose p,
+.prose ul,
+.prose ol,
+.prose blockquote {
+    max-width: 165mm;
+}
+
+/* Every section of the summary opens a page, so the space a heading keeps above it would only push
+   the page down. */
+.prose h1 {
+    margin-top: 0;
+}
+
+.prose ul,
+.prose ol {
+    margin: 0 0 3mm 0;
+    padding-left: 5mm;
+}
+
+.prose li {
+    margin-bottom: 1.4mm;
+}
+
+.prose li > p {
+    margin: 0;
+}
+
+.prose a {
+    color: var(--ink);
+    text-decoration: underline;
+    text-decoration-color: var(--border);
+}
+
+.prose blockquote {
+    margin: 0 0 3mm 0;
+    padding-left: 3mm;
+    border-left: 0.8pt solid var(--border);
+    color: var(--muted);
+}
+
+.prose pre {
+    background: var(--label-fill);
+    border: 0.4pt solid var(--border);
+    padding: 2mm;
+    white-space: pre-wrap;
+    margin: 0 0 3mm 0;
+}
+
+.prose hr {
+    border: none;
+    border-top: 0.4pt solid var(--border);
+    margin: 5mm 0;
+}
+
+/* The severity colour, where it marks a score in a table, is on the cell itself. */
+.grid td.sev {
+    white-space: nowrap;
+}
+
+.cover-intro {
+    margin-top: 9mm;
+}