perf(stream): make unknown-field emptiness checks O(1)

OTP 28 medians: rejecting 10,000-key objects fell from 4.36 to 1.68 ms (2.6x faster), with ordinary text unchanged. Preserve empty-value semantics; all 203 Gleam tests passed.

9137b328 parent 34b54dc1 +40 −12
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