Base v1.5.0

Added

• Added BASE_PLATFORM runtime metadata so Linux distribution-family support
  can be represented without overloading the coarse BASE_OS=linux contract.
• Added Ubuntu/Debian source-checkout CI validation and documented the
  accepted Ubuntu 24.04 ARM64 Parallels validation path.
• Added Linux-aware basectl check and basectl doctor prerequisite
  diagnostics for Python venv support, Git, GitHub CLI, BATS, ShellCheck, jq,
  and Go on Ubuntu/Debian.

Changed

• Centralized setup/check platform dispatch so macOS, Ubuntu/Debian, unknown
  Linux, and unsupported platforms are routed through explicit platform-policy
  helpers instead of scattered command conditionals.
• Changed Ubuntu/Debian setup behavior to fail conservatively with manual
  prerequisite guidance until full apt-backed bootstrap support is implemented.
• Updated compatibility, Linux-support, IDE-boundary, and forge-boundary docs
  to reflect Ubuntu/Debian runtime support and the remaining non-goals.

Fixed

• Scrubbed activate override variables in the source-checkout test harness so
  local shell state does not leak into BATS validation on Ubuntu or macOS.