Corgi advance: keep a chunk's columns when nothing reorders

[frankmcsherry]

CorgiChunk::advance did three things per call that it mostly didn't need to:

• concatenated its carried trailing group with the new chunks, a copy of every row;
• gathered every surviving row into new key and value columns, a second copy;
• sorted each group's rows by time, one group at a time.

In scc's churn rounds only 3% of groups need re-sorting after advancing (138k of 4.4M, counted).

Now advance works on whole chunks:

• A chunk is complete once the next chunk starts a new (key, val) group, or on done. Until then the last chunk waits, rather than being split to hold back its last group.
• Chunks that one group spans are concatenated and advanced together, as the old code did for everything.
• After advancing, a group whose times still ascend is left as it stands. When no group needs re-sorting, the chunk keeps its key and value columns: no copy, no gather.

One file, 72 lines added, 69 removed. Stands alone; no overlap with #902.

Corgi, 3 runs each, ms (load, then churn per round):

case	master-next load	this load	master-next churn	this churn
count	83.4–84.9	83.4–84.9	3.250–3.256	3.030–3.034
count2 (churn 10k)	87.3–88.7	87.3–88.8	22.78–22.80	21.66–21.74
min	86.1–86.6	86.1–87.6	2.546–2.569	2.394–2.418
reach	686–690	642–647	69.26–69.36	65.22–65.34
scc	674–680	654–681	32.89–33.14	31.46–31.59

Validation: interactive tests pass, including advance_owned_and_shared_nested_times_matches_reference, which feeds groups split across 1- and 3-row chunks. AoC 2023 passes 33/33 on corgi. The same-plan harness's output digests match compiled DD for count, count2, distinct, min, min2, reach and scc, at 1 and 4 workers.

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