Chunk pack: combine a carried run once, when emitted

[frankmcsherry]

pack (the maximal-packing helper behind Chunk::settle) grew its carry with combine(&mut acc, next). That is linear only when appending leaves acc in place, as Vec does. Corgi's combine rebuilds its columns, so a carry absorbing many small chunks recopied itself every time. Over scc's churn rounds that was 30.7M carried rows recopied to append 2.3M new ones (counted).

Now the carry is a run of chunks, and combine(run) is called once, when the run is emitted. Between calls the run goes back to the input uncombined. The three implementations change to match: vec concatenates, columnar melds each trie onto the first, and corgi calls its existing concat. The peel-an-oversized-chunk path is folded into the loop, so pack_absorb goes. 46 lines added, 71 removed.

One trade: a run handed back between calls is re-walked by the next call. That costs one deque pop and push per chunk, not a copy of its rows.

Churn, ms/round, corgi, 3 runs each (loads unchanged):

case	master-next	this
count	3.265–3.278	3.187–3.190
count2 (churn 10k)	22.87–22.90	22.38–22.44
min	2.574–2.578	2.523–2.540
reach	69.61–69.78	68.95–68.99
scc	33.19–33.35	32.87–33.11

Validation: DD lib tests plus the int_proxy, columnar, reduce_oracle, trace and scc tests pass, as do all interactive tests. AoC 2023 passes 33/33 on both vec and corgi. columnar_spill spills the same 5425 chunks at the same rate.

🤖 Generated with Claude Code