ci: rehearse Base Bash v2 pins

[codeforester]

Summary

• pin the main macOS and Ubuntu Base Demo validation jobs to the immutable Base v2 migration commit
• pin every base-bash-libs checkout to the current immutable v2 contract commit
• update validation contracts, docs, and changelog for the prerelease rehearsal

Issue

Closes #212
Related: basefoundry/base-bash-libs#240

User impact

Base Demo now validates the merged v2 base_ API and current base-bash-libs v2 contract without relying on moving branches or historical v1 release pins. The workflow still makes no v2 GA claim; Homebrew remains gated on the canonical release asset.

Validation

• ./tests/validate.sh
• bats tests/demo_test.bats tests/install_test.bats
• git diff --check
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash /Users/rameshhp/work/base/bin/basectl help

Notes

The source-provider compatibility job remains pinned to its explicit Base provider-alignment revision while consuming the current v2 library commit.