thiserror finalize: drop anyhow from production, add satkit::Error façade

[ssmichael1]

Summary

Final PR in the anyhow → thiserror migration tracked in #81. Closes #81.

Four commits in order:

1. f3c36b5 — Phase 4: typed errors for utils::download and utils::update_data
2. cf6a4c1 — Retype placeholder String/anyhow::Error variants now that all dependencies are migrated
3. 56a7ca1 — Move anyhow to [dev-dependencies]
4. 692efb2 — Add top-level satkit::Error façade

What changed

Phase 4 — utils::download::Error + utils::update_data::Error

• Both modules migrated. Stubs now return typed download::Error::FeatureDisabled / FileNotFoundNoDownload { path }.
• Five upstream modules (spaceweather, solar_cycle_forecast, earth_orientation_params, jplephem, earthgravity) now consume Download(#[from] download::Error) directly. The bridge From<anyhow::Error> impls are deleted.

Retyping

• tle::Error::InvalidEpoch(String) → #[from] InstantError
• tle::Error::KeplerConversion(String) → Kepler(#[from] kepler::Error)
• orbitprop::Error::Precompute(String) → Jplephem(#[from] jplephem::Error)
• New sgp4::Error (with SatRecInit(i32) + Source(Box<dyn Error + Send + Sync>) slot) — wasn't strictly in the brief but was needed to fully eliminate anyhow from sgp4_impl.rs and the SGP4Source trait.
• New frametransform::Error for to_gcrf/from_gcrf and IERSTable parsing — same reason.
• Doctests updated to Ok::<(), satkit::omm::Error>(()) style.
• Python bindings updated for the typed signatures.

anyhow move

• anyhow is now in [dev-dependencies] only. Production code references it nowhere.
• grep -rn "anyhow" src/ → 11 hits, all inside #[cfg(test)] mod tests blocks (test-only, kept for ergonomics).

satkit::Error façade

• New src/error.rs with 19 variants (one per public module Error). All #[error(transparent)] + #[from].
• pub use error::{Error, Result}; from lib.rs. Also re-exports InstantError for symmetry with Instant.
• update_data::Error variant gated #[cfg(feature = "download")] (matches the module's gating).

fn do_thing() -> Result<(), satkit::Error> {
    let tle = satkit::TLE::from_url(url)?;            // tle::Error
    let states = satkit::orbitprop::propagate(...)?;   // orbitprop::Error
    Ok(())
}

Judgment calls worth a look

1. tle::Error::Sgp4(String) left as String — sgp4::Error does impl std::error::Error now, so retyping is mechanically possible. Kept as String to limit surface change in this PR; trivial follow-up if you want it.
2. sgp4::Error::Source is Box<dyn Error + Send + Sync> rather than enum variants for OMM/TLE — adding From<omm::Error> for sgp4::Error would create an awkward inverse dependency (omm already depends on sgp4 for the trait). Boxed slot keeps the dependency arrow flowing one way.
3. download::Error has two stub-related variants (FeatureDisabled and FileNotFoundNoDownload { path }) — download_if_not_exist stub needs to convey "file missing AND no download feature available", which is semantically distinct from the other stubs that are pure feature-gate refusals.
4. Façade variant naming uses per-word capitalization for readability: JplEphem, SpaceWeather, EarthOrientationParams, SolarCycleForecast, EarthGravity, LpEphem, Frametransform.
5. Test-module anyhow imports kept (per the brief). 11 use anyhow::* lines inside #[cfg(test)] blocks.

Test plan

• cargo build — clean
• cargo build --no-default-features — clean
• cargo build --features download — clean
• cargo test --release — 158 lib + 39 doc pass
• cargo test --release --no-default-features — 157 lib + 38 doc pass
• cargo test --release --features download — 159 lib + 41 doc pass
• cargo check --manifest-path python/Cargo.toml — clean

Closes #81.

🤖 Generated with Claude Code