v0.8.0

v0.8.0 — thrust inputs, the system handle, four platforms, and correct covariances

Second stable release of the empyrean distribution. Everything the
0.8.0 release candidate carried, plus the two covariance-accuracy
fixes that validating the candidate uncovered — neither ever shipped
in a stable release with wrong behavior.

Features

• Continuous-thrust inputs across every channel: finite-burn arcs with
  constant-RTN, velocity-tangent, and inertial-fixed steering, plus
  delta-v targeting corrections whose covariances flow into the tagged
  per-epoch covariance.
• A reusable system handle: build_system assembles the force model
  once and reuses it across propagate / generate_ephemeris — shared
  across threads, with a describe() provenance record carrying the
  force-model menu and the SHA-256 identity of every loaded kernel,
  and loud identity/frozen-key/staleness guards.
• Sigma-point results are tagged with their own covariance kind
  (sigma_point) in the per-epoch readback instead of reading back as
  linear.
• Explicit kernel-load error categories.

Accuracy

• B-plane 3-sigma ellipses for element-space input orbits (the SBDB
  common case) no longer inflate by orders of magnitude: the
  projection uses the propagated Cartesian covariance at each close
  approach, never a native-representation covariance folded through
  the Cartesian STM.
• Sigma-point covariances use the canonical 2N+1 unscented
  construction; the previous sampling under-estimated them ~6x.
• 4-character observatory codes are rejected loudly at every input
  boundary instead of silently aliasing a different observatory.

Packaging & docs

• One cp310-abi3 wheel per architecture (CPython 3.10+), prebuilt for
  macOS arm64 / x86_64 and Linux x86_64 / aarch64; macOS x86_64 is
  cross-compiled on arm64 runners.
• Documented ordering guarantees (propagation states are
  epoch-ordered; ephemeris entries are observer-input-ordered within
  each orbit), mag_sigma population conditions, and the observation
  Jacobian's light-time caveat.
• Binds empyrean-core v0.8.1 (villeneuve v1.17.0, scott v1.13.2).