Adopt Fold anti-slop rules and Effect design guidance

[K-Mistele]

Summary

• tailor the Effect program-design guidance to Fold, including Predicate and Match policy
• vendor and enforce selected anti-slop and TypeOnce OXLint rules with explicit TUI exclusions
• replace silent error swallowing and chained ternary dispatch with clearer Effect APIs

Validation

• bun run lint
• bun run typecheck
• bun run test
• bun run format:check