mermaid-render: licensing: add LICENSE-MPL-2.0 for resvg/usvg distribution

[hartsock]

Phase: 6 • Branch: mermaid-render/6.2-mpl-license

What

Add LICENSE-MPL-2.0 at the crate root (or vendor it). Required for any binary/wheel distribution built with --features png (resvg + usvg are MPL-2.0). Document in COMPATIBILITY.md's licensing section.

Files

• scrybe-mermaid-render/LICENSE-MPL-2.0 (new)
• scrybe-mermaid-render/COMPATIBILITY.md (cross-reference)
• scrybe-mermaid-render/README.md (note the dual-license requirement)

Tests

• N/A (legal artefact); CI: cargo about (if used elsewhere) clean.

Acceptance

• LICENSE-MPL-2.0 present and referenced from README + COMPATIBILITY.md.

Refs

#37 gating checklist box "Licensing".

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