docs(spec): HTML companion for canonical examples spec

Per Alex's request: a self-contained HTML companion to the .md spec
with simplified mockups (inline HTML/CSS "screenshots") and flow
diagrams (HTML/CSS boxes + arrows; mermaid was considered but
rejected — inlining mermaid.js would bloat the file by ~2 MB, and
hand-authored flow blocks are clearer + theme-consistent).

The .md remains the canonical source of truth (Obsidian Sync only
propagates Markdown; HTML is regenerable from the .md). Both files
coexist with the same basename.

Color tokens lifted from the writing-specs-as-html house template.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: apiad