chore: cycle group defense in depth

[ledwards2225]

Adds safety mechanisms to protect against two currently unreachable cycle_group footguns:

• In-circuit rejection of point-at-infinity operands in cycle_group::_unconditional_add_or_subtract: the ecc_add_gate is degenerate at (0, 0) and admits forged outputs. Replaces builder-only BB_ASSERT(!is_constant_point_at_infinity) with in-circuit is_point_at_infinity().assert_equal(false). No additional gates and no VK change for existing circuits. Unreachable from noir.
• Clarify offset-generator collision-avoidance contract in MSM helpers: adds a BB_ASSERT(base_point != offset_generator) in straus_lookup_table to catch the (highly improbably) honest-dev mistake of these two coinciding (and makes it clear that they must not).