Let i be the loop variable when it is named as one (#976)

[Rafael-SOWNet]

Addresses the first half of #976. The second half is answered in a comment below rather than implemented, with the references you asked me to check.

i as the loop variable

sum(i, i, 1, 10) summed nothing. i is the imaginary unit, and that is decided in the lexer — NUMBER: ... | 'i' — so it never reaches the rule that makes variables and cannot be one anywhere in the language. The index arrived as a number, the operator had no variable to bind, and the whole expression stayed unevaluated. #966 documented that as a trap; you asked for it not to be one.

	before	after
sum(i, i, 1, 10)	unevaluated	55
product(i, i, 1, 5)	unevaluated	120
sum(i ^ 2, i, 1, 4)	unevaluated	30

Naming i as the index is a statement about the whole operator, so it is honoured through the summand and the bounds, not only in the index position. Doing it there alone would have been worse than leaving it broken: the index would become a variable while every i in the summand stayed the imaginary unit, nothing would substitute, and sum(i, i, 1, 10) would answer 10i — a wrong answer replacing an unevaluated one.

And only inside the operator that declares it. These are unchanged, and they are what make the above a fix rather than a new defect:

sum(i * k, k, 1, 3)   ->  6i
sum(k + i, k, 1, 2)   ->  3 + 2i
sum(i, i, 1, 3) + i   ->  6 + i
i * i                 ->  -1
sqrt(-1)              ->  i

Why this is in MathS, not the grammar

My first attempt was a grammar action. It fixed "sum(i, i, 1, 10)".ToEntity() and left MathS.Sum("i", "i", 1, 10) broken — one library with two answers, since the implicit string conversion is the only thing that goes through the parser. Putting it at the factory covers both paths, and leaves the parser completely untouched, so there is no regeneration and nothing for #965's grammar-drift check to catch.

Tests

The class remark on SummationProductTest taught the trap and now records that it is gone. The test that pinned the old answer is replaced by cases for both halves — the index being read, and i still being the imaginary unit everywhere else.

Suite 7318 passed, 0 failed, 14 skipped.