BUG - Spin angle should not be averaged across L1B blocks in GLOWS L2

[maxinelasp]

Parent issue: #1276

In DailyLightcurve.__post_init__ (glows_l2_data.py:86):

self.spin_angle = np.average(l1b_data["imap_spin_angle_bin_cntr"].data, axis=0)

The bin center angles are fixed for the observation day (same 3600 bins every block). Averaging them is unnecessary and potentially misleading. Use the first (or any single) timestamp's values directly instead of averaging across all timestamps.

Additionally, per the algorithm document (Section 10.6), position_angle_offset_average should be set to ψ_G,eff = 360° - ψ_GLOWS (a single computed value, not an average over the day), and position_angle_offset_std_dev should always be 0. The current L1B code (glows_l1b_data.py:855-857) correctly sets std_dev to 0, but the L2 code (glows_l2.py:121-127) re-averages all *_average fields including position_angle_offset_average, which is incorrect. The position angle offset should be passed through from L1B unchanged, not re-averaged.

[maxinelasp]

This is closely related to #2719 and proably should be completed at the same time. I'm already working on #2719, so I can take this ticket.