v0.7.0

Added

• SGP4/SDP4 orbit propagation. A full, dependency-free implementation of the
  standard simplified-perturbations propagator — near-Earth SGP4 together with the
  deep-space SDP4 extension (lunar-solar secular and periodic perturbations and
  12 h / 24 h geopotential resonance). It is validated against the official AIAA
  2006-6753 ("Revisiting Spacetrack Report #3") verification vectors: all 666
  reference states across the near-Earth, deep-space, resonant, and error-code
  cases match to a worst-case position error of about 4 mm. This is the model
  two-line element sets are defined against, so it represents real constellations
  — notably the ~12 h GNSS orbits, which are deep-space and resonant and which the
  earlier two-body + J2-secular model cannot capture.
• A constellation given as full two-line element sets (line 1 + line 2) is now
  propagated with SGP4/SDP4; a constellation given as line-2-only elements keeps
  the analytic Keplerian two-body propagation, unchanged. The two forms can be
  mixed in one block. New orbit-sgp4-gps.toml reference scenario (a GPS-like MEO
  constellation in real two-line format, propagated with SGP4) — drop in a current
  Celestrak "gps-ops" set to study the live constellation.

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Full changelog: CHANGELOG.md · Docs: README