perf(table): decode partition and sort fields once

[fallintoplace]

Summary

• Decode partition and sort field objects into one auxiliary value.
• Preserve presence checks for source-id, source-ids, and transform.
• Avoid the temporary raw map and second full-object decode.
• Add single-source and multi-source benchmarks.

Benchmark

Apple M1 Pro, arm64:

Decoder	Case	Before	After	Result
Partition	Single source	4,254 ns, 1,344 B, 30 allocs	2,168 ns, 976 B, 19 allocs	49% faster
Partition	Multi source	4,467 ns, 1,408 B, 34 allocs	2,735 ns, 1,032 B, 23 allocs	39% faster
Sort	Single source	3,508 ns, 1,360 B, 31 allocs	2,234 ns, 1,000 B, 20 allocs	36% faster
Sort	Multi source	3,977 ns, 1,424 B, 35 allocs	2,627 ns, 1,048 B, 24 allocs	34% faster

Bytes and allocations drop by about 26% and 31 to 37%.

Tests

• go test ./...
• go vet .