feat(clarity): Clarity content provider for state diagrams

[joshprice]

Summary

Adds a Clarity content provider that surfaces a live Mermaid state diagram on any Ash resource vertex whose module uses AshStateMachine. Same Mermaid as mix ash_state_machine.generate_flow_charts, but rendered in the Clarity UI directly from the DSL — no need to regenerate .mmd files.

Once merged, a consumer with both ash_state_machine and clarity in their deps automatically gets a State Machine tab alongside Resource Overview, Policies, Actions, etc. — zero config.

How it works

1. lib/clarity/state_machine_diagram.ex defines AshStateMachine.Clarity.StateMachineDiagram, guarded by with {:module, _} <- Code.ensure_loaded(Clarity.Content) so the module is only compiled when Clarity is present.
2. mix.exs registers it via application/0:
   ```elixir
   env: [clarity_content_providers: [AshStateMachine.Clarity.StateMachineDiagram]]
   ```
   Clarity's Clarity.Config.list_content_providers/0 walks every loaded app's env and picks it up automatically.
3. {:clarity, \"~> 0.3\", optional: true} added to deps — follows the same convention ash_diagram uses.

Diagram rendering delegates to AshStateMachine.Charts.mermaid_state_diagram/1, so the visual is guaranteed to match the DSL.

Rationale

Cross-cutting Ash diagrams (ER, class, C4) belong in ash_diagram. Extension-specific diagrams like state-machine flow are owned most naturally by the extension itself — the DSL and the diagram are one code change away from each other, versions move together, and no central-library coordination is required as the Ash ecosystem grows. Module name uses AshStateMachine.Clarity.* (rather than ClarityContent.*) to leave room for future Clarity additions (vertex types, introspectors) under the same namespace.

Test plan

• mix compile — passes
• mix test — 19 existing tests still green
• Verified locally in a demo project: when {:clarity, ...} is added, /clarity shows a "State Machine" tab on any resource that uses AshStateMachine, rendering the correct Mermaid diagram
• Verified the provider's applies?/2 correctly skips resources that don't use AshStateMachine
• CI

Notes

• Clarity 0.4.0's published igniter constraint (~> 0.6.25) excludes the igniter 0.7.x currently in mix.lock. mix deps.get after deleting the lock resolved cleanly; CI should confirm. If it doesn't, happy to pin clarity to ~> 0.4 or adjust the igniter constraint — whichever you prefer.
• Documentation: added a brief "Clarity integration" section to documentation/topics/charts.md.

[zachdaniel]

🚀 Thank you for your contribution! 🚀