fix(base-foundation, base-io): add toString() to Exceptional and PathSet

[deer]

Exceptional had no toString() override, so instances printed the default Object identity hash instead of anything useful for debugging or logging; this adds a toString() that renders Exceptional[value], Exceptional[exception], or Exceptional.empty depending on which of the three states the instance is in, matching the same three-way branching already used by equals() and hashCode().

PathSet's empty singleton previously rendered as the string "PathSet.empty()", which reads like a factory-method reference rather than a value representation and doesn't match how a populated PathSet prints. This adds a toString() to the internal non-empty PathSetBuilder implementation that delegates to the underlying LinkedHashSet<Path>, and changes PathSet.empty()'s toString() from "PathSet.empty()" to "[]" so both the empty and non-empty cases render consistently with each other and with standard Set formatting.