A[3:0] ──┐
B[3:0] ──┼──► CSel Adder ──┬──► S[3:0]
Cin    ──┘                 └──► Cout

Block 0 (bits 0-1): Ripple-carry with actual Cin
  └──► produces C1 (carry into bit 2)

Block 1 (bits 2-3): Pre-compute BOTH possibilities
  Path A: assume Cin=0 → S2_0, S3_0, Cout_0
  Path B: assume Cin=1 → S2_1, S3_1, Cout_1

Final: MUX selects Path A or B based on C1

Ripple-Carry:  [FA0]──►[FA1]──►[FA2]──►[FA3]  (4 stages)

Carry-Select:  [FA0]──►[FA1]──┐
               [HA2]──►[FA3]  ├──►[MUX]  (3 stages)
               [FA2]──►[FA3]──┘

threshold-carryselect-adder/
├── model.safetensors
├── create_safetensors.py
├── config.json
└── README.md