Conversion plan for lowering the ex dialect to func/arith/scf/cf.

M1 lowering for the scalar subset of the ex dialect:

• ex.lit -> arith.constant
• ex.add -> arith.addi
• ex.sub -> arith.subi
• ex.mul -> arith.muli
• ex.call -> func.call
• ex.return -> func.return
• ex.func -> func.func (body region moved, argument and ex.return types converted)

The ex term types (!ex.dyn/!ex.bound/!ex.unbound) convert to a scalar word type (i64) until the Zig term runtime lands.

plan/1 returns a reusable Beaver.MLIR.Conversion.Plan; run it with Beaver.MLIR.Conversion.Plan.run/2 and continue with the standard arith-to-llvm and func-to-llvm passes.

ex.var/ex.bind must be materialized to SSA first with Beaver.MLIR.Dialect.Ex.MaterializeBoundVariables.