Cover runtime bashrc startup behavior

[codeforester]

Summary

• Add runtime bashrc tests for non-interactive rcfile handling, missing ~/.baserc behavior, full debug trace propagation from ~/.baserc, and repeated sourcing.
• Make runtime bashrc startup idempotent so sourcing the rcfile twice does not rerun user startup.

Validation

• bats lib/bash/runtime/tests/runtime_bashrc.bats
• env HOME=/private/tmp/base-review-home BASE_TEST_PYTHON=/Users/rameshhp/.base.d/base/.venv/bin/python bin/base-test
• git diff --check

Closes #298