Split SYNDROME state into SYNDROME_S1 (parity computation) + SYNDROME_S2
(popcount) pipeline stages. SYNDROME_S2 uses a 4-level balanced adder tree
(224→56→14→4→1) instead of a serial accumulator loop, eliminating the
loop-carried dependency that Yosys could not optimize. This reduces the
critical path from ~48 ns to ~14 ns, achieving WNS=0.0 at TT corner (50 MHz).
Verilator verified: 2/2 basic + 20/20 vector tests pass.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>