fix!: keep unavailable command outcomes neutral

[eitsupi]

Summary

• distinguish successful, failed, and unavailable command outcomes without replacing R's native REPL
• keep prompt, history, history-forget, and sponge state neutral when error tracking is unavailable
• preserve and chain the startup options(error) handler through lexical closure capture while respecting later user changes
• avoid evaluating active bindings or writing active/locked tracking bindings across the Rust/R boundary
• evaluate the internal setup code in the base environment so user workspace masking cannot affect it

R compatibility

Safe binding inspection uses R_existsVarInFrame, which became public in R 4.2. The symbol is loaded as an optional capability rather than a requirement for loading libR.

Capability	Behavior
R_existsVarInFrame available	Install the error wrapper and provide full success/failure tracking
R_existsVarInFrame unavailable, including standard R versions older than 4.2	Do not install the wrapper; leave outcomes unavailable, history exit statuses NULL, the prompt neutral, and history-forget/sponge state unchanged

This is a breaking change for command outcome tracking on R versions without that API, but it does not establish R 4.2 as arf's minimum supported R version. Normal R semantics take priority over outcome accuracy.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --workspace --locked -- -D warnings
• cargo test --all-targets --all-features --workspace --locked
• interactive product test with R 4.0.5 and R 4.1.3: no tracking state or wrapper is installed, errors return to the prompt, and later evaluation succeeds
• interactive product test with R 4.4.3: the wrapper is installed, failures update the prompt, and evaluation continues afterward