idgen_array() allocates the next whole 32768 block and returns its
first element (array tag, index 1), refusing n outside 1..32767 and a
block past the 20-bit sequence (-EOVERFLOW, context unchanged). The
reset-on-new-second step is shared with idgen() via idgen_tick_locked();
idgen() itself is unchanged until task 9.1.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
