fix(r1cs): route wide shifts at the bit level

[NullWitnessZK]

Fixes #242.

Minimal reproducer

Save as main.pl:

main(A: private u128, B: private u128, R: public u128) :-
    A << B = R.

Use:

A = 0xffffffffffffffffffffffffffffffff
B = 127

Compare the two public results:

source-correct R = 0x80000000000000000000000000000000
R1CS-accepted R  = 0x2c425bfd0001a40100000000ffffffff

At CirC 271f911, raw and optimized CircIR accept only the first value. R1CS lowering and the finalized proof relation reject it and accept the second. A fresh stock proof verifies for the wrong public result.

Expected

low128((2^128 - 1) << 127) = 2^127.

Actual

The relation computes the low 128 bits after scalar-field reduction.

Why it happens

The lowering packs the 255-bit shifted intermediate into one BLS12-381 scalar before decomposing it. The integer exceeds the scalar modulus, so it is first reduced modulo the field.

Impact: wrong relation—both soundness and completeness.

Proposed fix

fix/u128-variable-shift at e47a41b uses an LSB-first bit-routing barrel shifter and never packs the oversized intermediate into a field element.

Self-contained regression:

cargo test --features r1cs target::r1cs::trans::test

The full focused R1CS translation group passes all 18 tests, including u128 SHL, LSHR, and ASHR.