v1.21.0: architecture decisions in the Spec tab

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Firefox: click markdown-pr-sentence-comments-1.21.0.xpi below and allow github.com to install the add-on.

Chrome, Edge, Brave: download markdown-pr-sentence-comments-chrome-1.21.0.zip, unzip it to a folder you keep, open chrome://extensions, switch on Developer mode, and click Load unpacked. For an update, replace the folder contents and click the reload icon.

New in this version:

• The Spec tab shows architecture-decisions.md as its own section between the proposal and the specs, one row per ## D1., ## D2. decision, with change dots and thread counts. The stats line counts the decisions.
• A decision heading that is not numbered in order gets a warning.
• Two more spec checks: a REMOVED requirement needs a **Reason** and a **Migration**, and a Purpose needs at least 50 characters.
• design.md stays supported for changes written with an older schema.

Based on chienyuanchang/rich-diff-comments (MIT).