Harden Mermaid rendering, SVG delivery, and label colors

[mjc]

Summary

• Make Mermaid's post-sanitization render boundary safe: errors are written as text, malformed or forged placeholders are rejected, source and complexity are capped, and comment threads share an aggregate render budget.
• Serve SVG attachments as inert downloads with a non-active content type while retaining normal inline behavior for safe image and media formats.
• Apply a safe fallback to invalid legacy, imported, or restored label colors before they reach frontend CSS sinks.

Regression coverage

• Mermaid limit tests keep ordinary diagrams renderable while rejecting oversized and dense sources, enforce aggregate byte and block limits, and confirm rejected input does not consume the shared budget.
• Mermaid render-boundary tests prove forged complex placeholders and malformed encoded sources never reach the renderer, multiple blocks share one budget, cancelled renders cannot mutate detached content, valid diagrams still render, and failures remain inert text.
• The label-color helper test preserves valid hexadecimal colors and replaces CSS source with the default color.
• The component rendering test feeds an unsafe stored color through the actual color picker and confirms neither the original value nor an injected CSS property reaches its rendered style.
• The attachment response test uploads an SVG containing script, then verifies it is returned as an attachment with an inert content type and restrictive content policy.

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