perf(compaction): compute survivor minimum once

[fallintoplace]

Summary

• Compute the survivor minimum sequence once per decision.
• Avoid repeating the partition-minimum scan for every equality-delete candidate.
• Work changes from O(P x D) to O(P + D).
• Add a scaling benchmark.

Benchmark

Apple M1 Pro, arm64. 1,000 partitions and 1,000 equality-delete candidates:

	Before	After
Time	14.999 ms/op	65.7 us/op
Bytes	25,223,048 B/op	96,144 B/op
Allocs	9,006 allocs/op	15 allocs/op

That is about 228x faster, with 99.6% fewer bytes and 99.8% fewer allocations.

At 10,000 partitions and 1,000 candidates, time drops from 149.7 ms/op to 254 us/op.

Tests

• go test ./...
• go vet .