Standalone test (test/test_de_reader.c): generates a synthetic 12KB
DE binary with known Chebyshev coefficients, exercises header parsing,
polynomial evaluation at 5 domain points, Earth derivation via
center=99, Moon geocentric, layout validation, range/body error paths,
NAN sentinel, and garbage file rejection. 55 tests, no PG dependency.
Makefile: add `make test-de-reader` target; also includes the earlier
sat_code C++ -> pure C sgp4 migration that was missed from prior commits.
astro_math.h: document that ecliptic_to_equatorial/equatorial_to_ecliptic
are NOT safe for aliased (in-place) calls — equ[1] is written before
equ[2] reads ecl[1]. The vendored sgp4/sdp4.c has separate in-place
versions using a temp variable.
