feat: Add Mermaid namespace clustering support

[kerrizor]

Summary

Add cluster: true support for Mermaid classDiagram output, grouping entities by their Ruby namespace into Mermaid namespace { } blocks.

This brings Mermaid to parity with the existing Graphviz clustering feature.

Closes #479

Motivation

When working with large schemas that include multiple namespaces (e.g., Admin::, Billing::, SolidQueue::), it's helpful to visually group related models together. The Graphviz generator already supports this via the cluster option, but Mermaid output ignored it.

Changes

• Add namespace grouping logic to lib/rails_erd/diagram/mermaid.rb
• Entities grouped by namespace into Mermaid namespace { } blocks
• Nested namespaces converted to dot notation (Admin::Users → Admin.Users)
• Entities without namespace appear outside any namespace block
• Relationships render outside namespace blocks (Mermaid requirement)
• Warning emitted when cluster: true used with erDiagram (not supported)
• Add 7 new tests covering all clustering scenarios
• Document the cluster option in README.md

Example Output

# .erdconfig
mermaid_style: classdiagram
cluster: true

Produces:

classDiagram
    namespace Admin {
        class User
        class Role
    }
    namespace Billing {
        class Invoice
        class Payment
    }
    User --> Role
    Invoice --> Payment

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Limitations

Clustering is not supported with Mermaid's default erDiagram style (ER diagrams don't have a namespace concept). A warning is emitted if cluster: true is used with erDiagram.

Testing

• 391 tests pass (0 failures)
• 7 new tests added for clustering scenarios
• Edge cases verified: nested namespaces, cross-namespace relationships, specializations