(fix): USE_POOLING=false path fixes and 2-tier toggle rename

[mgyoo86]

Summary

Fix bugs in the USE_POOLING=false code path and clarify the 2-tier pooling toggle naming.

Changes

Bug fixes

• Make DisabledPool <: AbstractArrayPool — fixes type checks failing under USE_POOLING=false
• Add missing reshape! fallback for DisabledPool{:cuda}
• Fix @maybe_with_pool :backend function form ignoring MAYBE_POOLING_ENABLED[]

Refactor: 2-tier pooling toggle rename

• USE_POOLING → STATIC_POOLING (compile-time const, master switch)
• MAYBE_POOLING_ENABLED → MAYBE_POOLING (runtime Ref{Bool}, @maybe_with_pool only)
• Old names kept as simple const aliases for backward compatibility
• LocalPreferences.toml key use_pooling unchanged

Tests

• Significantly expanded USE_POOLING=false separate-process tests (290+ lines)
• Added @maybe_with_pool :backend macro expansion tests
• Added CUDA DisabledPool tests

Breaking changes

None — existing code using USE_POOLING and MAYBE_POOLING_ENABLED continues to work.