Configure features

Declare project features and defaults in besa.toml:

[features.build-source]
default = true
kind = "build"

[features.toolchain-cpp]
default = true
kind = "toolchain"

[features.toolchain-cuda]
default = false
kind = "toolchain"

[features.user-docs]
default = false
kind = "documentation"

Every selectable project capability is a feature. kind is metadata; selection still uses one feature namespace.

The CMake backend receives feature overrides through PROJECT_FEATURES:

cmake -S . -B workspace/build \
  -DPROJECT_FEATURES='toolchain-cuda;~toolchain-cpp'

Positive entries enable a feature. ~feature disables a default feature. Each underlying feature may appear at most once; contradictory or repeated entries are rejected.

Features with kind = "toolchain" are compilation-context features. BESA's CMake backend maps the reserved toolchain-* names it supports to CMake languages and enables those languages only after the complete feature configuration has been resolved. Toolchain-specific compiler options and architecture choices remain toolchain/backend configuration rather than fields in besa.toml.