Add PX7 Markdown Viewer tool page

[px7digital]

Summary

Add a tool page and icon for PX7 Markdown Viewer, a free native macOS app that renders local Markdown files in the app and Finder Quick Look and can edit the plain-text source.

Disclosure: I maintain the app. The page is intentionally factual, identifies the app as proprietary, and records unsupported syntax instead of presenting it as a full Markdown editor.

Verification

• All 28 syntax IDs match _data/tools.yml, with no missing or duplicate rows.
• The public product and distribution-repository links return HTTP 200.
• Renderer checks cover headings, GFM tables, task lists, strikethrough, fenced Swift highlighting, autolinks, escaped raw HTML, and the absence of footnote semantics.
• The icon is the current application icon resized to 512×512.

The page deliberately marks footnotes, heading IDs, definition lists, emoji shortcodes, highlight, subscript, superscript, raw HTML, and disabling autolinks as unsupported.