The naga MSL backend forwarded the WGSL `coopLoad(T)` / `coopStore(T)`
`row_major` flag straight through to Metal's `simdgroup_load` /
`simdgroup_store` `transpose` argument. Metal's `transpose` means
"memory is transposed relative to the simdgroup_matrix canonical layout"
— and on Apple silicon that canonical layout is row-major, so
`transpose=false` is the correct load for row-major memory. Naga's IR
uses `row_major=true` to mean "memory is row-major"; the emission must
therefore negate the flag when targeting Metal, matching the SPIRV
backend which already correctly maps `row_major=true → RowMajorKHR`.

Symptom: any WGSL using `coopLoadT` / `coopStoreT` on row-major memory
produced numerically-wrong results on Apple silicon. The existing
wgpu cooperative-matrix example masked this because it used `coopLoad`
on row-major memory (the matching error cancelled the MSL one on
Metal), and because its A-matrix init happened to be symmetric so the
corresponding bug on Vulkan was also invisible. Neither regression
path will be silent after this commit:

 - `naga/src/back/msl/writer.rs` emits the correct `transpose` flag for
   `coopLoad` / `coopStore` and `coopLoadT` / `coopStoreT`. The
   snapshot in `naga/tests/out/msl/wgsl-cooperative-matrix.metal` is
   regenerated accordingly (2-line diff).
 - `examples/features/.../shader.wgsl` now uses `coopLoadT` /
   `coopStoreT` so that the WGSL matches the "Row-major tiles"
   comment in the surrounding host code. This also serves as a working
   reference for the naga convention.
 - `examples/features/.../mod.rs` reinitialises matrix A with a
   row/column-weighted formula so neither A nor B is symmetric in
   (i, j); the test can no longer pass by coincidence regardless of
   which load variant a future regression picks.
 - `docs/api-specs/cooperative_matrix.md` documents both variants
   (`coopLoad` = column-major, `coopLoadT` = row-major), updates the
   worked example to use the T variants with its row-major buffers,
   and adds "variant does not match storage" as an undefined-behaviour
   case.