Add base_init bootstrap smoke tests

[codeforester]

Summary

• Add dedicated BATS smoke tests for the base_init.sh runtime path, host, OS, shell, idempotency, and import_base_lib contracts.
• Normalize BASE_OS to Base contract values such as macos and linux.
• Export BASE_SHELL=bash when base_init.sh is sourced outside an already-marked runtime shell, while preserving the existing runtime-shell sentinel.
• Wire tests/base_init.bats into bin/base-test and CI.

Validation

• bats tests/base_init.bats
• bats lib/bash/runtime/tests/runtime_bashrc.bats
• ./bin/base-test
• git diff --check

Fixes #303