Split the Remux and Container Steps Diagram into Narrower Charts #1107

Behavior

The ARCHITECTURE.md "Remux and Container Steps" section draws one flowchart TD holding six unconnected flows: the Convert.ReMuxToMkv() subgraph, RemuxByExtension(), RemuxNonMkvContainer(), RemuxRemoveExtraVideoTracks(), RemoveCoverArt(), and RemoveTags(). Mermaid lays separate top-down flows out side by side. The chart therefore renders very wide, and once scaled down to fit the page the text is too small to read.

Fix direction

Unfold the chart into several smaller charts, one per step or one per small group of related steps, so each chart gets the full page width:

• Convert.ReMuxToMkv() on its own, since three steps call it.
• RemuxByExtension(), RemuxNonMkvContainer(), and RemuxRemoveExtraVideoTracks(), either together or each on its own. Each ends on a reference to the remux, rather than drawing the subgraph again.
• RemoveCoverArt() on its own, which is the longest flow.
• RemoveTags() and RemoveAttachments() together.

Give each chart a short lead sentence naming the step it draws. Keep the node text, the suspect class markings, and the S-number references that "Suspect Branches" links to unchanged, so that only the layout moves. Confirm that each chart renders readably on GitHub at normal page width.

Check the other sections in ARCHITECTURE.md for the same pattern of several unconnected flows in one chart. If any has it, unfold those charts the same way in the same pull request.