Fix method overwriting error during precompilation on Apple Silicon

Resolves #768 by using Union type for fallback init_cacheval methods

The issue was that individual fallback methods for BLISLUFactorization,
CudaOffloadLUFactorization, and MetalLUFactorization had the same signatures
as their extension counterparts, causing method overwriting during precompilation
when LinearSolveAutotune is loaded.

This change consolidates the three individual fallback methods into a single
method using a Union type, which avoids the signature conflict while maintaining
the same functionality.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

src/factorization.jl

@@ -1203,19 +1203,9 @@ function init_cacheval(::CliqueTreesFactorization, ::StaticArray, b, u, Pl, Pr,
     nothing
 end
 
-# Fallback init_cacheval for extension-based algorithms when extensions aren't loaded
-# These return nothing since the actual implementations are in the extensions
-function init_cacheval(::BLISLUFactorization, A, b, u, Pl, Pr,
-        maxiters::Int, abstol, reltol, verbose::Bool, assumptions::OperatorAssumptions)
-    nothing
-end
-
-function init_cacheval(::CudaOffloadLUFactorization, A, b, u, Pl, Pr,
-        maxiters::Int, abstol, reltol, verbose::Bool, assumptions::OperatorAssumptions)
-    nothing
-end
-
-function init_cacheval(::MetalLUFactorization, A, b, u, Pl, Pr,
+# Fallback method for extension-based algorithms when extensions aren't loaded
+# Uses Union type to avoid method overwriting with extension-specific methods
+function init_cacheval(::Union{BLISLUFactorization, CudaOffloadLUFactorization, MetalLUFactorization}, A, b, u, Pl, Pr,
         maxiters::Int, abstol, reltol, verbose::Bool, assumptions::OperatorAssumptions)
     nothing
 end