Function repairMermaidBlock

Repair pipeline used by the orchestrator. Applies the safe fixes in the order that yields the most converging behaviour observed during the corpus scan:

1. quadrant label parens (lexical-error class A)
2. quadrant data-point float normalisation (lexical-error class B)
3. timeline section parens (parse-error class A)
4. flowchart node label quoting (parse-error class B)
5. invalid style / classDef / linkStyle purge (chart-type-rejects-style class)
6. edge-label closing-bracket typo (-->|x] → -->|x|)
7. unsupported diagram replacement (last resort)