Bump version to 0.51.0 due to propagate changes to downstream consumers

## Breaking API changes (AI Generated)

- **`ObjectPool` moved** (#975): now lives in `diskann-utils`. Update
imports from `diskann::...::ObjectPool` → `diskann_utils::ObjectPool`.

- **`AlignedSlice` removed** (#994): the `AlignedSlice` abstraction in
`diskann-vector` is gone. Code that converted between vector
representations through `AlignedSlice` should now use the `Poly` /
`CastFromSlice` polymorphic interfaces directly (see
`diskann-vector::conversion` and `diskann-quantization::alloc::poly`).
Storage that previously held `AlignedSlice` values should hold `Poly<T,
A>` instead.

- **`AsThreadPool` generic removed** (#967): functions that previously
took `pool: impl AsThreadPool` now take `pool: &RayonThreadPool`. Pass a
borrow of an existing pool; remove the generic parameter from your call
sites.

- **`sgemm()` returns `Result`** (#997): in `diskann-linalg`, the new
signature is:

  ```rust
  pub fn sgemm(
      atranspose: Transpose, btranspose: Transpose,
      m: usize, n: usize, k: usize,
      alpha: f32, a: &[f32], b: &[f32],
      beta: Option<f32>, c: &mut [f32],
  ) -> Result<(), SgemmError>
  ```

`SgemmError` has variants `InvalidMatrixDimensions { matrix_name,
expected_rows, expected_cols, actual_len }` and `DimensionOverflow {
matrix_name, rows, cols }`. Replace previous panic-on-bad-input
assumptions with explicit handling.

- **Benchmarks are stateful** (#995): the `Benchmark` impls in
`diskann-benchmark` are no longer stateless unit structs. Each benchmark
type now has a `::new()` constructor (often holding `PhantomData<T>` or
plugin state), and registration uses an instance:

  ```rust
  // before
  benchmarks.register("name", MyBench);
  // after
  benchmarks.register("name", MyBench::<T>::new());
  ```
If you wrote a custom benchmark, give it a `new()` and register an
instance. Combined with #996, search-side benchmarks now compose
`Plugins<Provider, Phase, Strategy>` and expose builder methods like
`.search(plugin)` to register search plugins on the instance.

- **`diskann-benchmark`: `async` → `graph-index`** (#1009): the
benchmark category previously named `async` was renamed to
`graph-index`. JSON config `type` values and example file names changed
accordingly:
  - `async-build` → `graph-index-build`
  - `async-dynamic-run` → `graph-index-dynamic-run`
- and the same prefix swap for `*-pq`, `*-sq`,
`*-spherical-quantization`, etc.

Update any benchmark config files, scripts, or CI that reference the old
`async-*` names.

- **`diskann-disk` buffer alignment decoupled from `block_size`**
(#984): code that assumed I/O buffer alignment equals the disk block
size should now configure alignment explicitly.

## Non-breaking

- New cache-aware block-transposed Chamfer/MaxSim distance for f32/f16
(#863).
- A/A benchmark documentation (#974); CI publish workflow improvements
(#755, #1017); openssl bump (#973); `compute_closest_centers` allocation
reduction (#980).
- **`DistanceComputer` `'static` bound relaxed** (#1007) and **redundant
`DistanceFunction` impls removed** (#1008)


**Full Changelog**:
v0.50.1...v0.51.0