fix(scs): don't panic on a recorded addition with a zero coefficient

[0xShadowX]

Compiling this with the SCS builder fails with parse circuit: div by 0, while R1CS and the test engine accept it:

t1 := api.Add(c.A, c.B, api.Neg(c.B))
t2 := api.Add(api.Mul(2, c.A), c.B, api.Neg(c.B))

reduce merges B and -B into a single term with coefficient 0, so the first Add records A*1 + B*0. The second one finds that entry in addConstraintExist, the q3*q2 == q1*q4 check passes as 0 == 0, and then Inverse(q2) fails and hits the panic("div by 0") // shouldn't happen branch. #1068 fixed the same kind of zero coefficient for Mul, but the Add path can still get one through cancellation.

The fix returns "not found" when q2 has no inverse, so the builder just adds a new constraint for this (rare) case. Nothing changes for circuits that compiled before.

Tests:

go test -count=1 -run TestExistDiv03 ./frontend/cs/scs/
go test -short ./frontend/... ./internal/stats/...

TestExistDiv03 fails on master with div by 0 and passes with this change; the stats test shows no constraint count changes.

• regression test added
• no generated files touched