bug: accept v2 prerelease base-bash-libs versions during runtime bootstrap

[codeforester]

Problem

Base's v2 migration accepts a major-version-shaped BASE_BASH_LIBS_VERSION, then calls the strict dotted-numeric base_require_version 1.4.0 helper. The local Base Bash 2.0.0 RC bundle reports 2.0.0-rc.1, so basectl help fails during base_init.sh with:

base_require_version expects dotted numeric versions
Base requires base-bash-libs 1.4.0 or newer; loaded version is '2.0.0-rc.1'.

This blocks validating and adopting the v2 release candidate.

Goal

Make Base's runtime bootstrap parser explicitly accept the supported Base Bash v2 prerelease and GA version forms while retaining the existing v1.4+ behavior during the migration window. Reject malformed versions and unsupported major lines with a clear error.

Acceptance criteria

• 2.0.0-alpha.N, 2.0.0-beta.N, 2.0.0-rc.N, and 2.0.0 load successfully.
• Existing 1.4.0+ stable-version behavior remains covered.
• Malformed prereleases and unsupported major versions are rejected without invoking the strict numeric parser on an invalid value.
• Focused Base Bats coverage and the full Base test suite pass.
• basectl help succeeds when BASE_BASH_LIBS_DIR points at the local 2.0.0-rc.1 review bundle.