Implement annual stellar aberration (Meeus Chapter 23)

[MattBlack85]

Adds src/aberration.rs with annual_aberration(jd, ra_deg, dec_deg) -> (Δα", Δδ")
following Meeus equations 23.3–23.4. The Sun's apparent longitude, true
obliquity, eccentricity and perihelion longitude are derived from the
low-precision solar theory in Chapter 25.

Four unit tests are included:

• Meeus Example 23.a (θ Persei, 2028 Nov 13.19): Δα≈+30.045", Δδ≈+6.681"
• Equatorial star at J2000.0 — physical range check
• Southern high-declination star on 1987 Apr 10 — range check
• Near-polar star — verifies Δδ stays bounded and Δα is finite

Also adds TODO.md tracking completed and future algorithm work.