Release 3.0.0 RC2

Changes from 3.0.0-rc1 to 3.0.0-rc2

• blosc2_get_slice_nchunks(), schunk_get_slice_nchunks(), and
  b2nd_get_slice_nchunks() now return int64_t instead of int.
  This removes an artificial INT_MAX limit on the number of chunks
  reported for large slices. Because these public signatures changed,
  this is an API/ABI break and callers should be rebuilt against 3.0.0.

• The internal parallel execution model has been reworked around a shared
  managed thread pool. Instead of owning a private worker pool per
  compression/decompression context, contexts now attach lazily to a pool
  shared by other contexts with the same nthreads setting, while
  caller-managed callback threading remains supported as before. This greatly
  reduces redundant thread creation and idle thread accumulation when many
  contexts coexist, improves scalability and resource usage, and fixes
  reliability problems in downstream workloads such as python-blosc2 that
  create large numbers of arrays/contexts over time. The new queue-based
  scheduling model also supports true concurrent submissions to the same pool.

• Landed a broad set of robustness and security hardening fixes in frame,
  schunk, lazy-chunk, metadata, mmap, and getitem paths. These changes tighten
  bounds checking, reject malformed headers/offset tables/VL-block metadata more
  aggressively, prevent integer overflows and out-of-bounds reads/writes, and
  add regression tests for malformed inputs and edge cases. Thanks to
  @metsw24-max for many of these.

• Modernized codec dependency handling in CMake. lz4, zlib-ng, and
  zstd are now resolved either from external packages (when preferred and
  available) or via FetchContent using pinned upstream versions, instead of
  being built from vendored in-tree copies. The optional ZFP plugin is now
  also obtained via FetchContent too. As a result, blosclz is now the only
  codec still vendored in-tree.

• Added explicit CMake cache variables for pinned codec versions and local
  source overrides:
  BLOSC_LZ4_VERSION, BLOSC_ZLIBNG_VERSION, BLOSC_ZSTD_VERSION, and the
  matching BLOSC_*_SOURCE_DIR variables.

• Improved CMake install/export support for static builds so downstream
  find_package(Blosc2) consumers keep working when fetched codec libraries
  are embedded into the Blosc package.

• Embedded third-party headers installed by the CMake package are now placed
  under a Blosc-owned include subtree (blosc2/thirdparty/...) instead of the
  top-level include directory, reducing the risk of header name collisions.

• Replaced deprecated exec_program() usage in cmake/FindSIMD.cmake with
  execute_process(), avoiding warnings with newer CMake versions.

• Removed the unmaintained Intel IPP integration. C-Blosc2 no longer exposes
  a CMake option to enable/disable IPP, does not probe for IPP at configure
  time, and always uses the maintained native codec paths instead.