M2: Fixture System (litmus-rz7q)

Create fixtures/ directory at workspace root. Each fixture has setup.sh, command.sh, optional teardown.sh, README.md. Initial fixtures: git-diff, ls-color, cargo-build, shell-prompt, git-log, python-repl, htop. Add fixtures/README.md with authoring guide.

Summary of Changes

Created fixtures/ directory at workspace root with 7 fixture subdirectories:

• git-diff/ — real git diff with additions, deletions, context (setup: git repo with committed files + unstaged changes)
• ls-color/ — real ls -la with dirs, symlinks, executables, hidden files
• git-log/ — real git log –graph with branches, merges, tags (pinned timestamps for reproducibility)
• cargo-build/ — real cargo build with intentional warnings + type error
• shell-prompt/ — scripted bash session output (interactive shell not scriptable reliably)
• python-repl/ — scripted Python REPL session output
• htop/ — top -b batch mode with scripted fallback

Each fixture has setup.sh + command.sh (all tested working). fixtures/README.md with authoring guide. Interface: FIXTURE_WORK_DIR env var, CWD set to work dir when command.sh runs.