Improve how @tldraw/mermaid converts and integrates diagrams

order them by importance, figure an issue with sub issues and list them

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This tracks improvements to how @tldraw/mermaid turns mermaid source into tldraw shapes. The sub-issues are ordered by importance: problems that silently break apps using the package come first, then problems that make converted diagrams wrong or hard to read, then support for more diagram types. Within a tier, problems already seen in a converted diagram rank above ones only found by reading how the conversion works. Several came out of fixing sequence destroy, note sizing, font-size directives and self-loops in #10750, #10753 and #10773, and the existing open mermaid issues are included so the package has one place to track its work.

Each sub-issue says whether its problem has been seen in a converted diagram or only found by reading the code.

Confidence: 85%, ready to get started.