fix: make resolver error handling iterative

[duriantaco]

Summary

Backport upstream resolver fix that makes resolver error handling iterative instead of recursive.

This avoids recursive traversal while constructing resolver error reports, reducing stack risk for deeply nested incompatibility causes.

Upstream

• 6feeacc62 avoid recursive resolver error handling

Tests

cargo fmt --check
cargo test -p fyn --test it --no-run
cargo clippy -p fyn --test it -- -D warnings