Epic: Migrate to Formal MLIR Rust Crate (`melior`) for Hardware Plugin IR Generation

The current plugin architecture uses lightweight, string-based mocking for MLIR generation (Operation, Module structs in hardware_trait.rs). To fully comply with the formal VxHardwarePlugin contract, we must replace this mock strategy with robust, AST-based MLIR generation utilizing a proper Rust bindings crate (such as melior).

Rationale

Relying on string concatenation for IR generation does not scale to complex hardware topologies and hinders modular optimization passes. A formal MLIR crate ensures type safety, predictable lowering, and standard validation before binary generation.

Subtasks

• Investigate and integrate melior (or another suitable MLIR Rust binding library) into the Vx build system.
• Refactor codegen.rs to construct an actual MLIR Context, Module, and Operations using the library's AST instead of String accumulation.
• Update VxHardwarePlugin trait to pass native melior::ir::Operation pointers.
• Migrate AppleNPEPlugin (and any other reference plugins) to utilize the formal MLIR inputs for their vendor-specific passes and lowering.
• Ensure the JIT execution (lli) and mlir-opt pipeline interacts correctly with the dynamically produced MLIR.

Start with hardware_trait.rs and codegen.rs to understand the current string-based Operation and Module flow, then inspect the Vx build system and AppleNPEPlugin integration. Review how lli and mlir-opt are currently invoked. Done means the formal MLIR crate is integrated, plugins use native IR inputs, and the JIT and optimization pipeline still interact correctly with generated MLIR.