alignment: fix plugin selection persistence, sanitize polar sync entries

[ckemper67]

Problem

Three independent bugs that each corrupt pointing after a restart or a single near-pole sync:

1. Plugin selection not persisting. saveConfig(true, name) only patches the named property in the existing XML; it skips CURRENT_MATH_PLUGIN. After a restart the text property is replayed before the switch property, so the selected external plugin (Nearest, SVD, …) is overwritten by the stale built-in default.

2. Mount alignment lost on plugin switch. When loading a new plugin, SetApproximateMountAlignment was never called on it. Every external plugin defaults to ZENITH, silently switching EQ drivers to the AltAz code path and producing garbage Goto coordinates whenever the user changes plugins at runtime.

3. Degenerate sync points near the celestial pole. When all sync points are close to the celestial pole (Dec near +/-90), the HA component of the telescope direction vector is unconstrained — any HA value maps to nearly the same point on the sky. All plugins that solve a least-squares system over the sync database (BuiltIn, SVD, Nearest) are affected: the solver assigns an arbitrary HA-axis correction that produces wildly wrong Gotos away from the pole. The same singularity occurs for altaz mounts when all sync points are near the zenith.

Fix

• telescope_simulator.cpp: use saveConfig() (full save) instead of saveConfig(true, name) so CURRENT_MATH_PLUGIN is included.
• MathPluginManagement.cpp: capture ApproximateMountAlignment before unloading the old plugin, then call SetApproximateMountAlignment + Initialise on the newly loaded one. Also skip a redundant reload when the switch property arrives after the text property on startup.
• MathPlugin.cpp / MathPlugin.h: add SanitizePolarEntries(), called from Initialise(). Entries with |Dec| > 88 (EQ) or encoder El > 88 (altaz) are shifted to 80 while applying the same delta to the paired celestial coordinate, preserving the pointing correction while breaking the degeneracy. The TDV is rebuilt using RA-based encoding, consistent with the BuiltIn/SVD/Nearest convention.
• ccd_simulator.cpp, guide_simulator.cpp: fix a unit mismatch — IEquatorialCoordinates.rightascension is in degrees, not hours; remove the erroneous x15 / /15 factor in the EQUATORIAL_PE snooping path.

Tests

Six new PolarDegeneracy tests in test_alignment_plugins cover EQ and altaz single-point-near-pole scenarios for BuiltIn, SVD, and Nearest plugins.