beam_search: stabilize tie handling and per-shot state on symmetric instances

[aleverrier]

This PR stabilizes beam decoding on symmetric instances (observed on Gross [[144,12,12]]).

Changes in decoder/src_cpp/beam_search.hpp:

• replace <=0 with <0 in sign/tie checks
• add deterministic micro-jitter (~1e-10) to initial LLRs
• reset decode output state at start of each shot
• add decode_calls counter used for deterministic jitter seeding

Result:

• removes the p=0.05 syndrome-failure/non-convergence spike in our Gross-code tests.