Rust error design

Convert ad-hoc error handling to typed errors. The wrong shape leaks through public APIs and infects every caller, so this refactor is usually worth doing early.

Decision tree

Is this code a library / reusable module (has consumers who may need to
match on error variants, retry, or wrap)?

├── YES → thiserror. Define a concrete error enum per module.
│         Use #[from] for upstream errors so `?` composes cleanly.
│         Never return Box<dyn Error> or Result<T, String>.
│
└── NO  → It's a binary / application top layer.
          anyhow::Result<T> with .context("...") at each layer.
          Use anyhow::bail!() / ensure!() for ad-hoc errors.
          Match on root_cause() / downcast_ref() only at boundaries.