…rk buffers

Remove the mutable keyword from comm_workbuf_float_ and
comm_workbuf_double_ by switching from std::vector (which returns
const T* from const data()) to std::unique_ptr<T[]> (whose get()
returns T* from const method).

Key changes:
- Pre-allocate work buffers in allocate_comm_buffers() called from
  getstartgr(), using the already-computed numr/startr/numg/startg
  arrays to determine the maximum required buffer size
- acquire_comm_workbuf<T>() no longer resizes lazily; it returns the
  pre-allocated buffer via unique_ptr::get() with an assertion guard
- Add cleanup in destructor via unique_ptr::reset()

Rationale: unique_ptr::get() is a const method that returns a
non-const T*, matching the semantic intent — a const PW_Basis
does not re-seat the buffer pointer, but the pointed-to scratch
memory remains mutable for MPI write operations. This avoids the
thread-safety concerns of mutable while maintaining const-correctness
throughout the gather/scatter call chain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>