Add accTitle/accDescr accessibility directives to all diagram types

[theletterf]

Summary

• Add AccessibilityInfo model and AccessibilityParser pre-processor
• Parse accTitle: ..., accDescr: ... (single-line), and accDescr { ... } (multi-line)
• SVG output includes role="img", aria-roledescription, aria-label, <title>, <desc>
• Works across all 13 diagram types
• Backwards compatible: no attributes when directives are absent

Changes

• AccessibilityInfo.cs — new model record
• AccessibilityParser.cs — shared pre-processor with Extract() method
• MermaidRenderer.cs — calls parser, passes info to renderers
• StyleBlock.cs — AppendSvgOpenTag accepts accessibility info, emits ARIA attributes
• All 11 SVG renderer files — updated signatures to pass through accessibility info
• AccessibilityTests.cs — 24 new tests

Test plan

• All 438 tests pass (414 existing + 24 new)
• All 13 diagram types tested with correct aria-roledescription
• Single-line and multi-line accDescr
• HTML character escaping in titles
• No regression when directives are absent

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