Range analysis: add symbolic constant folding for SymbExpr and cover identity simplification cases

[Copilot]

SymbExpr simplification was missing constant-identity folding, so symbolic bounds retained avoidable noise (x+0, x*1, x-0) and equivalent forms. This PR adds constant-aware simplification rules and extends symbolic range coverage to assert those expressions collapse as expected.

• Symbolic simplification: constant-aware folding

  • Added constant helpers in SymbExpr (const_bits, zero/one checks).
  • Added try_simplify_constants() and integrated it into simplify() before linear flattening.
  • Implemented identity/annihilator rewrites for core ops:
    • x + 0, 0 + x → x
    • x - 0 → x
    • x * 1, 1 * x → x
    • x * 0, 0 * x → 0 (explicit zero-constant selection)
    • x / 1 → x
    • x & 0, 0 & x → 0
    • x | 0, 0 | x → x
    • x ^ 0, 0 ^ x → x
    • x << 0, x >> 0 → x
    • -(-x) / !(!x) → x

• Symbolic range fixture updates

  • Added const_identity_case in range_symbolic to exercise +0, *1, -0 paths.
  • Updated test_symbolic_interval expectations to validate simplified symbolic output and guard against unsimplified identity forms.

fn const_identity_case(x: i32) -> i32 {
    let y = x + 0;
    let z = y * 1;
    let w = z - 0;
    w
}