fix(geometry): preserve Angle.angle_value across become()

[Chessing234]

Changelog / Overview

Angle.get_value() returned the initial angle even after the defining lines moved, when the angle was wrapped in always_redraw.

Cause

always_redraw(lambda: Angle(l1, l2)) refreshes the mobject via become(), which copies points/submobjects but not the cached angle_value attribute that get_value() returns.

Fix

Override become() on Angle to copy angle_value from the source Angle.

Testing

l1 = Line(LEFT, RIGHT); l2 = Line(DOWN, UP)
a = always_redraw(lambda: Angle(l1, l2))
l1.rotate(PI/4); a.update()
a.get_value()   # before: 1.5708 (stale); after: 0.7854

Fixes #4512

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