Corgi reduce: merge unit-valued runs without a sort

[frankmcsherry]

merge_present merges presented chunk runs straight into the bridge when they arrive in ((key, value_id), time) order, and otherwise consolidates. It recognized leaf values as ordered but not Unit values, though all unit rows share one id. Unit-valued presentations (distinct's output, and unit-valued inputs such as reach's join(edges, ($2 ;)) | distinct) paid for a sort. The merge also reserves its bridge up front, as the fallback's extend does, rather than growing it by doubling.

The ordered_ids line is the same as in #870, which predates the IdPool refactor.

Same-plan harness, 1 worker, median of 3 (before -> after):

reach     2M edges, 1000 x 100   load 767 -> 679 ms    churn 85.2 -> 76.4 ms/round
scc       200k edges, 100 x 50   load 674 -> 666 ms    churn 35.6 -> 35.4
distinct  1M rows, 1000 x 500    load 87.5 -> 84.9 ms  churn 2.51 -> 2.45
count     1M rows, 1000 x 500    load 88.5 -> 85.7 ms  churn 3.62 -> 3.54

Peak RSS: a 4M-row count or distinct load 1289 -> 1145 MiB (the reserve); reach 609 -> 640 MiB, not traced.

Validation: outputs match compiled DD on the harness cases (count, count2, distinct, min, min2, reach, scc) at 1 and 4 workers; workspace tests pass.

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