Base-point calculations for the length-two cross exception #
This file develops the low-degree base-point criterion used in the corrected
distinct-strand theorem. It is kept separate from the dependent semibreak
update API in LengthTwoCross.
In the low-degree range, a length-two midpoint already present in the semibreak part is a base point of the corresponding banana normal form.
Adding a chip to a strand explicitly known to be empty is the inverse of
semibreakDivisor_remove_chip.
On a length-two strand, vanishing at the midpoint means that the strand slot is empty, so adding the midpoint chip preserves semibreakness.
The complementary rank computation: when the midpoint is absent from the semibreak part, the endpoint relation converts its subtraction into a normal form with both endpoint coefficients lowered.
A length-two strand has exactly one interior slot, so a semibreak divisor has either zero or one chip at its normalized midpoint.
Exact midpoint base-point dichotomy in the low-degree normal-form range. It is the local input for the corrected length-two cross exception.