Speed up integrator alt/az via pyerfa + rewritten FastAltAz

[brickbots]

Summary

The integrator's per-frame altaz/constellation work was dominated by skyfield's observe()/apparent()/altaz() chain, which the integrator called twice per frame (eyepiece + camera center). This PR replaces it with erfa.atco13 (pyerfa) — the same SOFA/ERFA precession/nutation/aberration math, ~19× faster, accurate to ~14″.

• Skyfield_utils.radec_to_altaz: reimplemented via erfa.atco13. Same (ra, dec, dt, atmos=True) signature. Adds pyerfa==2.0.1.5 to requirements.txt (small C-extension wheel, ARM-friendly).
• FastAltAz: body rewritten using IAU-1982 GMST + spherical trig + Bennett refraction. Fixes a subtle LST bug (the prior 0.985647 · days_since_j2000 term double-counted the fractional day) and adds the previously-missing refraction. API unchanged: __init__(lat, lon, dt) + radec_to_altaz(ra, dec, alt_only=False). Continues to back catalogs.py and calc_object_altitude.
• Skyfield_utils.set_location: now no-ops when (lat, lon, altitude) is unchanged, removing the per-tick wgs84.latlon rebuilds for callers like the integrator and chart UI.

Performance

Measured on macOS dev machine; Pi numbers will scale, but ratios should hold.

Path	before	after	speedup
Skyfield_utils.radec_to_altaz	~915 µs (skyfield)	~48 µs (pyerfa)	~19×
Skyfield_utils.set_location (same loc)	~25 µs	~0.25 µs (cached no-op)	~100×
FastAltAz.radec_to_altaz	~3.4 µs	~1.6 µs	~2×
Integrator per-frame altaz block¹	~1840 µs	~113 µs	~16×

¹ set_location + 2× radec_to_altaz + radec_to_constellation.

Accuracy

vs raw skyfield observe()/apparent()/altaz("standard") reference, median angular separation:

Path	median error
Skyfield_utils.radec_to_altaz (pyerfa)	14″ (~arcsec)
FastAltAz (rewritten)	~13′ (~0.3°, precession-dominated, same bar calc_object_altitude already lived with)

Test plan

• nox -s lint — clean (after auto-fixes of unused imports)
• nox -s format — clean
• nox -s type_hints — clean on this branch's changed files (only pre-existing errors elsewhere)
• nox -s smoke_tests — 2/2 pass
• nox -s unit_tests — 107/107 pass, including 9 new tests:
  • TestFastAltAz: bounds, alt_only short-circuit, refraction sign, accuracy floor vs skyfield, LST advances with time
  • TestSkyfieldUtilsRadecToAltaz: requires set_location, matches skyfield within 120″ (atmos=True) and 30″ (atmos=False) at well-elevated points, atmos flag lifts altitude
• Smoke test on a Pi to confirm the pyerfa wheel installs and runs (recommended before merging)

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