fix: preserve contradictory constant pin constraints

[NullWitnessZK]

Summary

Preserve an explicit contradiction when constant-pin simplification encounters different constants for the same signal.

Given:

signal x;
x <== a + b;
x === 3;
x === 5;

the source constraint system is unsatisfiable. Before this change, fast substitutions were inserted into a map keyed by signal, so the later x = 5 substitution overwrote x = 3. -O1 dropped one pin and -O2 could emit zero constraints.

Expected versus current behavior:

       current       expected
-O0    4 constraints  contradiction preserved
-O1    2 constraints  contradiction preserved
-O2    0 constraints  contradiction preserved

Fixes #425. The upstream issue contains the complete standalone reproducer and observed compiler output.

Changes

• Detect multiple substitutions for the same signal while encoding fast substitutions.
• If the constants disagree, preserve a contradictory constraint rather than overwriting the first substitution.
• Add a regression covering the conflicting-pin case.

Validation

cargo test contradictory_constant_pins_are_not_overwritten
git diff --check

The focused test passes. Repository-wide formatting was not applied because the upstream checkout has unrelated pre-existing formatting differences.