Combine row stepping and column collection

[bradengroom]

Depends on #1444. Incremental diff. I will rebase this once the dependency PR lands.

Summary

SQLiteRows.Next currently crosses cgo once to call sqlite3_step and, for every returned row, a second time to collect its column values. This PR combines both operations in one C helper.

It retains the same row buffer, conversions, error handling, and cancellation tracking; the helper collects values only when sqlite3_step returns SQLITE_ROW.

Benchmarks

Apple M4 Max, Go 1.26.5, 10 interleaved runs at 500 ms per benchmark. Medians were compared with benchstat; ~ means no statistically significant difference at p=0.05. Base is 840a3ec and this PR is 0869632.

Benchmark	Base	This PR	Change
RowsContext/rows=1/context=non_cancelable	994 ns	981 ns	-1.3%
RowsContext/rows=1/context=cancelable	1.146 µs	1.144 µs	~
RowsContext/rows=1000/context=non_cancelable	96.0 µs	77.7 µs	-19.1%
RowsContext/rows=1000/context=cancelable	104.3 µs	85.2 µs	-18.3%
BenchmarkSuite/BenchmarkQuery	1.854 µs	1.834 µs	-1.1%
BenchmarkSuite/BenchmarkRows	52.0 µs	50.2 µs	-3.5%
BenchmarkSuite/BenchmarkStmtRows	49.7 µs	47.2 µs	-5.0%

Bytes and allocations per operation are unchanged in every measured benchmark.

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[mattn]

#1444 has been merged — thank you again! Feel free to rebase this one onto master and mark it ready for review; the incremental diff looked promising and I'll benchmark it the same way (please keep an eye out for regressions on the non-cancellable path when fusing step and column collection).