feat: add chunk row accessors for typed and string reads

[mrchypark]

Summary

• add ChunkRows plus Conn.QueryChunksContext and Stmt.QueryChunksContext for chunk-by-chunk result access
• add typed DataChunk accessors for INTEGER, BIGINT, DOUBLE, TIMESTAMP, and zero-copy VARCHAR reads
• add tests and benchmarks covering chunk iteration, null handling, type mismatches, and chunk-oriented performance

Motivation

The existing database/sql row path is still the right default, but some consumers need a lower-overhead read path when they already process results chunk-wise. This change keeps the existing API untouched and adds an opt-in chunk API on top of the existing DataChunk abstraction.

The typed accessors are intentionally narrow:

• IsNull
• Int32Slice
• Int64Slice
• Float64Slice
• TimestampMicrosSlice
• StringRefs

All returned views are documented as valid only until the next NextChunk call or Close.

Benchmarks

On an Apple M1 with:

go test ./... -run '^$' -bench 'BenchmarkQueryChunksContext(TypedNumericScan|GetValueNumericScan|TypedStringScan|Count)$' -benchmem -benchtime=1x

• BenchmarkQueryChunksContextGetValueNumericScan: 37.57 ms/op, 1.06 MB/op, 131162 allocs/op
• BenchmarkQueryChunksContextTypedNumericScan: 4.66 ms/op, 13.9 KB/op, 346 allocs/op
• BenchmarkQueryChunksContextTypedStringScan: 13.51 ms/op, 1.07 MB/op, 444 allocs/op
• BenchmarkQueryChunksContextCount: 4.71 ms/op, 18.1 KB/op, 411 allocs/op

The main improvement is removing per-cell boxing and GetValue overhead when the caller can already consume chunk-oriented data.

Validation

• go test ./...
• go test ./... -run '^$' -bench 'BenchmarkQueryChunksContext(TypedNumericScan|GetValueNumericScan|TypedStringScan|Count)$' -benchmem -benchtime=1x

[mlafeldt]

I would appreciate any guidance on how you would like this to be shaped before a full review starts.

Thanks for the draft. I think the overall shape looks good. Keeping it as an explicit low-level, opt-in path is the right call. Please keep it focused. We can add more typed accessors later. Maybe add a test for empty and non-inlined strings as well.

[mrchypark]

I would appreciate any guidance on how you would like this to be shaped before a full review starts.

Thanks for the draft. I think the overall shape looks good. Keeping it as an explicit low-level, opt-in path is the right call. Please keep it focused. We can add more typed accessors later. Maybe add a test for empty and non-inlined strings as well.

Thanks, that makes sense. I'll keep this PR focused and add coverage for empty strings and non-inlined strings in the StringRefs tests.

[mlafeldt]

Thanks for this. A few things to fix before merge:

1. StringRefs crashes on NULL VARCHAR rows
2. IsNull doesn't validate rowIdx
3. TimestampMicrosSlice doc is inaccurate
4. CI fails: golangci-lint run reports 8 issues

See my inline comments.