Add safe uninitialized GEMM overwrite paths - #196

Summary

• Add explicit-context overwrite-only MaybeUninit<T> entry points for einsum2, dot-general, and raw batched GEMM.
• Route naive, system BLAS, and injected BLAS providers through beta-zero overwrite paths without reading the destination.
• Add checked output injectivity, conservative input/output overlap rejection, checked arithmetic, non-contiguous temporary/writeback handling, and feature-specific differential/regression tests.
• Migrate non-Faer strided-opteinsum intermediate pooling to overwrite-only storage while preserving owned-input paths.
• Raise cblas-inject to 0.1.2 for the exact-zero beta contract.

Faer boundary

Faer 0.24.4 does not expose a typed API that permits a MatMut<T> overwrite over MaybeUninit<T>. The Faer-only public uninitialized entry points therefore return EinsumError::Unsupported and retain the initialized compatibility path. This PR does not use an unsafe cast, early assume_init, or zero-fill workaround. The remaining Faer implementation is tracked in strided-rs#195.

This is the safe backend portion of strided-rs#188; it intentionally does not claim that the Faer portion is complete.

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test --workspace with CARGO_BUILD_JOBS=4 and linker threads limited to 1 to avoid the local Rust 1.97 linker resource failure
• strided-einsum2 feature matrix: no-default, Faer, BLAS, and blas-inject
• strided-opteinsum feature matrix: no-default, Faer, BLAS, and blas-inject
• injected BLAS fallback and poisoned-C overwrite testsnnRefs