feat: release Pinset v2.1.0 batch selection

[MoorZhou]

Summary

• allow pinset global and pinset use to accept variable-length batches across any supported Provider set
• resolve the complete batch before one atomic state update, preserve unmentioned selections, and install once in dependency order
• add failure-safety, compatibility, help, documentation, and version 2.1.0 release metadata

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked (304 tests)
• cargo check --workspace --all-features --locked
• integration contract tests
• Windows installer/uninstaller contract tests
• Unix uninstaller contract test
• shim dependency boundary check