feat: add MaybeUninit-safe overwrite matmul - #270

Summary

Add a typed write-only destination for overwrite-only matrix multiplication:

• add public MatUninitMut over MaybeUninit<T> storage;
• add matmul_with_conj_uninit with explicit Par;
• use the existing native gemm path with read_dst = false;
• use a raw-pointer scalar fallback without forming destination references before writes;
• return an initialized MatMut only after every logical output element has been written;
• cover column-major, row-major, strided/conjugated, empty-inner-dimension, Rayon, and non-native scalar paths.

The API preserves the existing initialized matmul entry points. A destination made with from_raw_parts_mut retains the existing raw-view safety preconditions, including exclusive borrowing, valid allocation, and non-overlapping logical elements.

Contract

The implementation does not read prior destination values in overwrite mode, does not use assume_init or zero-fill an output-sized temporary, and accepts explicit parallelism rather than ambient Rayon state. On panic, the caller retains MaybeUninit<T> storage and no initialized reference is exposed.

Downstream tracking: tensor4all/strided-rs#198
Upstream request: #269

Verification

• cargo test --workspace --quiet (Faer 197 passed, 2 ignored; faer-ffi 37 passed)
• cargo check --workspace --all-targets
• cargo check -p faer --no-default-features --features linalg
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps
• rustfmt check on all changed files
• git diff --check