fix(runtime): accept prerelease base-bash-libs versions

[codeforester]

Summary

• Make Base validate BASE_BASH_LIBS_VERSION before invoking the strict v1 dotted-numeric minimum-version helper.
• Accept v2 alpha, beta, RC, and GA forms while preserving the v1.4+ minimum check.
• Add regression coverage for accepted and malformed prerelease versions and make the fixture version assertion follow the active library fixture.

Issue

Closes #1882

Validation

• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats tests/base_init.bats (22 passed)
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CLI_SOURCE_DIR=/Users/rameshhp/work/base-cli/lib/python env -u BASE_HOME ./bin/base-test (944 pytest passed, 1 skipped; 852 Bats passed)
• BASE_BASH_LIBS_DIR=/private/tmp/base-bash-libs-rc-review.KKiigr/base-bash-libs-2.0.0-rc.1/lib/bash BASE_CLI_SOURCE_DIR=/Users/rameshhp/work/base-cli/lib/python env -u BASE_HOME ./bin/basectl help (passed)
• git diff --check (passed)

Demo Impact

None. This is a runtime bootstrap compatibility fix with regression coverage.

Notes

• The fix is Base-local; no compatibility shim or parser change was added to base-bash-libs.
• .ai-context/ unchanged: this preserves the existing runtime contract and only corrects prerelease parsing.