v0.1.0

• release: v0.1.0 — pin github.com/ubgo/cache v0.1.0
• ci: add task cover, coverage badge, and CI coverage floor gate
• test: raise coverage toward 100% with edge and error-path tests
• docs: add per-file context headers for AI onboarding
• docs: per-feature cookbook with use cases and snippets
• ci: drop obsolete sibling-checkout from test workflow
• Initial release: ubgo/cache-mem