fix: improve AssertIsCrumb readability and add constant check

[forkfury]

Add constant value check at the start of AssertIsCrumb to avoid unnecessary constraint generation for compile-time constants.

Use separate variable x instead of overwriting input parameter for better code clarity.
Add mathematical comment explaining the polynomial constraint derivation, matching the SCS implementation style.

---

Note

Optimize and clarify AssertIsCrumb by adding a constant fast path, cleaner polynomial constraint with auxiliary variable, and explanatory comment.

• Assertions (frontend/cs/r1cs/api_assertions.go):
  • AssertIsCrumb:
    • Add constant-value fast path: accept constants in [0,3], panic otherwise with clearer error message.
    • Rework constraint using auxiliary x := i1² - 3i1, enforcing x(x+2) = 0 without mutating the input.
    • Add brief mathematical comment explaining the polynomial derivation.

^{Written by Cursor Bugbot for commit 8a7ffdb. This will update automatically on new commits. Configure here.}