perf(arrow/csv): avoid materializing rows before writing

[fallintoplace]

Rationale for this change

Writer.Write currently converts columns to strings, then copies every string header into a row-major [][]string before calling WriteAll. This allocates one []string for every row. Large record batches spend a lot of memory on this temporary matrix.

What changes are included in this PR?

• Keep the converted strings in their existing column slices.
• Reuse one row-sized []string while writing each CSV record.
• Preserve the existing flush and error behavior.

For the existing 1,000-row, 16-column benchmark, medians from 6 runs on an Apple M1 Pro were:

	main	this PR	change
time/op	4.78 ms	4.61 ms	-3.5%
B/op	2,292,279	2,012,183	-12.2%
allocs/op	41,514	40,515	-2.4%

This removes 999 allocations per call for 1,000-row batches.

Are these changes tested?

• go test ./arrow/... -count=1
• go test ./arrow/csv -run "^$" -bench "^BenchmarkWrite$" -benchmem -count=6

Are there any user-facing changes?

No. The generated CSV output is unchanged.