remove leading lagged NA/NaN values for surd ts defination

[SpatLyu]

This PR updates the time-series handling in the surd implementation by removing leading NA values introduced by lag operations.

---

Changes

• Applied in the surd source code (time-series branch):

  • Condition: nb.isNull() and nrows.isNull()

• Remove the first |lag| observations from each result vector:

for (auto& vec : pm)
{
    vec.erase(vec.begin(), vec.begin() + lag_abs);
}

• Only applied when:

  • lag_abs > 0
  • lag_abs < n_obs

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Behavioral Notes

• Eliminates artificial leading NA values caused by lagging in surd
• No effect when lag == 0