Move Builder out of helpers.hpp into detail/builder.hpp:
- Extract the Builder class and declptr helper into a focused header
  that declares only the forward types it actually needs (item, group,
  h_item, id, nd_item, range). Device-side SPIR-V built-in access
  is kept self-contained via spirv_vars.hpp.

Move SPIR-V fence helpers out of helpers.hpp into
detail/spirv_memory_semantics.hpp:
- getSPIRVMemorySemanticsMask (memory_order and fence_space overloads)
  now lives in a header that only pulls spirv_types.hpp, access/access.hpp,
  and memory_enums.hpp.

Make detail/helpers.hpp a thin forwarder:
- Include builder.hpp + spirv_memory_semantics.hpp.
- Retain get_or_store<T> and is_power_of_two in-place.
- Drop the forwarding class/enum declarations now in builder.hpp.
- All existing #include <sycl/detail/helpers.hpp> sites continue to
  work without modification.

Split sycl/sub_group.hpp into focused detail headers:
- detail/sub_group_core.hpp: sub_group struct with lightweight query
  API (get_local_id, get_group_id, leader, etc.) fully inline, plus
  forward declarations for the deprecated load/store and barrier
  members. Includes only spirv_vars.hpp, access/access.hpp, the narrow
  fwd/multi_ptr.hpp forward header, id.hpp, range.hpp, and
  memory_enums.hpp. No spirv_ops.hpp, no bit_cast, no generic_type_traits.
- detail/sub_group_extra.hpp: out-of-line definitions of the deprecated
  barrier() and barrier(fence_space) methods. Includes spirv_ops.hpp
  and spirv_memory_semantics.hpp.
- detail/sub_group_load_store.hpp: the detail::sub_group namespace
  block-load/store helpers and the out-of-line definitions for the
  deprecated load/store member templates.
- detail/sub_group.hpp: internal aggregator (core + extra + load_store)
  for SYCL runtime and extension headers that need the full type.

Make sycl/sub_group.hpp a thin aggregator:
- Include detail/sub_group_core.hpp + detail/sub_group_extra.hpp +
  detail/sub_group_load_store.hpp + nd_item.hpp.
- Keep the out-of-line nd_item::get_sub_group() definition here.

Narrow ext/oneapi/free_function_queries.hpp:
- Include detail/builder.hpp and detail/sub_group_core.hpp directly
  instead of the heavier group.hpp + sub_group.hpp umbrella includes,
  avoiding the load/store machinery for a header whose only sub_group
  use is constructing a default sub_group().

Update include_deps tests to reflect the new include graph.

No ABI or API changes: all deprecated sub_group load/store and barrier
members are preserved with the same signatures. Existing
free_function_queries.hpp remain public entry points).

Compile-time impact on ext/oneapi/free_function_queries.hpp
-----------------------------------------------------------
Measured with clang -ftime-trace on a device-only SYCL compilation.
Transitive SYCL headers: 36 -> 32; stdlib headers: 17 -> 10.

Headers removed from the include closure of free_function_queries.hpp:
  sycl/sub_group.hpp          (replaced by sub_group_core.hpp)
  sycl/__spirv/spirv_ops.hpp  (was pulled by sub_group load/store)
  sycl/detail/generic_type_traits.hpp  (was pulled by SelectBlockT)
  sycl/detail/address_space_cast.hpp   (was pulled by dynamic_address_cast)
  sycl/bit_cast.hpp           (was pulled by block read/write casting)
  + 7 stdlib headers (utility, limits, initializer_list, and friends)
    driven out by the above.

Per-header isolated compile time (device-only, spir64):

  Header                                   PR: intel#21762      DEV       Delta
  free_function_queries.hpp (whole)            109 ms     71 ms    -38 ms (-35%)
  sycl/sub_group.hpp                           107 ms     n/a       eliminated
  sycl/__spirv/spirv_ops.hpp                    45 ms     n/a       eliminated
  sycl/detail/generic_type_traits.hpp           50 ms     n/a       eliminated
  sycl/detail/sub_group_core.hpp                  n/a     52 ms     new (replaces sub_group.hpp)

The layering also sets up a clean future deletion path: if/when the
deprecated load/store API is removed, the work reduces to deleting
sub_group_load_store.hpp, sub_group_extra.hpp, and ~18 declaration
lines in sub_group_core.hpp — no surgery on a 671-line monolith.