[WIP] Add support for dynamic dimensions in einx

[Copilot]

• Inspect repository structure and locate axis-solving and dynamic-shape related code paths
• Establish minimal-change implementation plan for dynamic dimensions
• Run baseline lint/tests to capture current status before edits
• Implement surgical solver/expression changes to allow unresolved dynamic axes
• Add focused tests for dynamic axis behavior and adapt_with_vmap use case
• Run targeted tests for changed areas
• Run code review + security scan and address findings
• Final verification and summary

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