reveal-md indents only each slide's first line (the heading) 12 spaces.
reveal.js's markdown plugin derives the dedent width from that line and
runs a global `\n? {12}(.*)` strip; since the leading newline is
optional, it also matched the ≥12-space alignment padding inside table
cells, splitting rows mid-line so marked couldn't parse the table.
Collapse the padding to single spaces (alignment colons preserved).

reveal.js 5.x emits `<code class="mermaid">` for ```mermaid fences;
renderMermaid still looked for the old `code.lang-mermaid` class, so no
diagram was ever found. The vendored mermaidAPI loads fine.