Added time-keeping and kernel downloading

data/kernels.tsv

@@ -0,0 +1,15 @@
+# ANISE kernels, pinned by URL and SHA-256. Fetch with scripts/fetch-kernels.sh.
+#
+# Not committed: together they are about 38 MB, and a checksum pins a file more
+# precisely than a copy of it does. The date is when the checksum was taken, so a
+# later mismatch can be told apart from a transfer error. Tab-separated; read by
+# both the fetch script and src/kernels.rs, so there is one list rather than two
+# that can disagree.
+#
+# NAIF's earth_latest_high_prec.bpc is deliberately NOT used. "Latest" is not a
+# thing a reproducible result can cite, so the dated snapshot covering the same
+# span is pinned instead.
+#
+# name	sha256	downloaded	url	purpose
+de440s.bsp	c1c7feeab882263fc493a9d5a5b2ddd71b54826cdf65d8d17a76126b260a49f2	2026-09-30	https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/de440s.bsp	Planetary and lunar ephemerides, 1849 to 2150. Sun and Moon positions for third-body and solar radiation pressure.
+earth_000101_261226_260929.bpc	df5510d1995ec56af62c2682affc38781e3aa4fda37722d2fa0a10e531b90b1d	2026-09-30	https://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck/earth_000101_261226_260929.bpc	Earth orientation, 2000-01-01 to 2026-12-26, reconstructed through 2026-09-29. Needed for inertial to Earth-fixed transformations.