Avoid boxing the BLAS-contractable inputs in _blas_contract!

[lkdvos]

Fixes #273, together with QuantumKitHub/Strided.jl#94, without inlining the call chain. Based on #314.

The 112 B per call in the reproducer of #273 (Julia 1.13.1) have two independent sources:

• 64 B here: makeblascontractable returns either its input or a permuted copy. With an allocator such as Bumper's, the copy has a different parent type, so the result is a union of two StridedView types. That union is not isbits and was boxed on every contraction, even when no copy was made. _blas_contract! now makes the inputs BLAS-contractable one at a time, and passes them on to the next step (_blas_contract_B!, then _blas_contract_C!) within each branch rather than returning them, so the union is never materialized. The copy itself is made by a small @inline helper: inlining it lets the compiler elide the Array wrapper of the copy, which otherwise costs an extra allocation per copy.
• 48 B in Strided.jl: variables captured by the Threads.@spawn closure in _threaded_blas_mul! were boxed, fixed in Avoid boxing the views captured by the threaded BLAS multiplication Strided.jl#94.

makeblascontractable itself is unchanged, since it is tested and possibly used elsewhere.

Benchmarks

Allocations in the reproducer of #273 (StridedBLAS, Bumper allocator), with the latest Strided and StridedViews main:

	main	this PR	this PR + Strided.jl#94
Float64	112 B	48 B	0 B
ComplexF64	112 B	48 B	0 B
ComplexF64, conj	48 B	48 B	0 B

Runtime and compile time (minimum times, fresh processes):

	main	this PR
reproducer, Bumper	422–434 ns	423–431 ns
same contraction, default allocator	1.83 μs / 16 allocs	0.45 μs / 4 allocs
8⁴ contraction requiring permuted copies, default allocator	12.7–13.7 μs / 25 allocs	8.9–9.6 μs / 7 allocs
first call, rank-5 ComplexF32 contraction	5.9 s	5.65 s
first call, Bumper contraction	32.6 s	32.7 s

Part of the speedup with the default allocator overlaps with the scalar type specialization of makeblascontractable in #314, since withblascontractable also specializes on it.

🤖 Generated with Claude Code

[lkdvos]

Folded into #314, since this fixes the Enzyme failures that the specialization of makeblascontractable in #314 triggered on Julia 1.13. Closing in favor of #314.