Add BASE_PLATFORM runtime metadata

[codeforester]

Summary

• Add readonly BASE_PLATFORM runtime metadata while keeping BASE_OS coarse.
• Classify macOS as macos, Ubuntu/Debian-family Linux as linux-debian, and other Linux distributions as linux-unknown.
• Document that BASE_DISTRO_ID and BASE_ARCH stay deferred until diagnostics or package selection need separate public axes.

Part of #562.
Fixes #1330.

Validation

• git diff --check
• shellcheck --severity=error base_init.sh bin/base-test lib/shell/baserc_guard.sh
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats --print-output-on-failure tests/base_init.bats cli/bash/commands/basectl/tests/update-profile.bats cli/bash/commands/basectl/tests/completions.bats
• env -u BASE_HOME BASE_CACHE_DIR=/private/tmp/base-1330-base-test BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test