kepler: validation, per-instance mu, w→argp, helpers (0.22, do not merge yet)

[ssmichael1]

Tracks #167. Parked until the 0.22 minor bump — do not merge. Rust API shape changes (field rename, new struct field, #[non_exhaustive] error enum).

Implements the remaining four items of the 2026-08-30 Kepler interface review (items 1–4 shipped in #146):

• (5) Validation — Kepler::try_new / Kepler::validate with kepler::Error::InvalidElement { name, value, reason }; the Python constructor and every element setter (a, eccen, inclination, raan, argp, nu, mu, deprecated w, and mean_anomaly / eccentric_anomaly) raise ValueError for a non-finite element, a <= 0, eccen ∉ [0, 1), incl ∉ [0, π], mu <= 0 (bounds strict; a failed setter leaves the set unchanged — no setter can leave NaN behind). Kepler::new stays unchecked. from_pv now raises ValueError (was RuntimeError) for open/rectilinear states and a bad mu; malformed input shape stays RuntimeError.
• (6) mu — Kepler.mu (m³/s², default MU_EARTH), with_mu, from_pv_with_mu; every dynamic quantity uses it. Python kepler(mu=), kepler.mu, from_pv(mu=). Pickle is 7 fields; serde defaults a missing mu to Earth's.
• (8) w → argp — Rust field renamed (only in-crate user: tle/fitting.rs). Python: argp canonical, same positional slot; w still accepted as constructor keyword and property with DeprecationWarning, kept indefinitely (no removal scheduled); both given → ValueError.
• (9) Helpers — periapsis, apoapsis, specific_energy, angular_momentum, flight_path_angle, argument_of_latitude, true_longitude; SatState::from_kepler / satstate.from_kepler; one-line repr; PartialEq + serde on Kepler, Copy/PartialEq on Anomaly.

Docs: docs/guide/kepler.md (argp, Validation and Derived-quantities sections, per-instance μ), stubs with units on every kepler docstring, CHANGELOG (Added + Breaking-Rust lines).

Testing

• Rust: 4 new unit tests (try_new rejection matrix incl. boundaries, MU_MOON period ratio + from_pv_with_mu round-trip, every helper against vis-viva / |r×v| / asin(r·v) identities and wrapping, serde round-trip + legacy-JSON mu default). Full suite green: 296 lib + all integration binaries (GMAT corpus included).
• Python: 6 new tests (pickle with mu, ValueError matrix + setter atomicity, argp/w equivalence + DeprecationWarning via pytest.warns, mu dynamics, helpers, repr + satstate.from_kepler). 197 passed, 1 skipped, warning-free. The NaN-no-hang regression test became test_anomaly_setters_reject_non_finite_and_high_e_converges: non-finite mean_anomaly / eccentric_anomaly → ValueError with the set unchanged, eccen = 1.5 rejected, and a capped e = 0.999 solve under a watchdog thread; test_kepler_from_pv_open_or_rectilinear_is_value_error covers the exception-type change.
• cargo clippy --all-targets -D warnings, cargo fmt, stubtest: clean.

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https://claude.ai/code/session_01PG969yapJ84DJceKt21Wen