recsplit: prepare for parallelism (#19514)

To introduce parallelism (in next PR) need first get-rid from "shared
mutable fields"

- `recsplit()` is now pure. it accepting in/out params:
-  `recsplitScratch` it's `in` parameter/bufs/configs
- `result` is `out` param/bufs
- write to files now happening outside of `recsplit()` func

---------

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

Author: AskAlexSharov

db/recsplit/golomb_rice.go

@@ -87,6 +87,31 @@ func (g *GolombRice) Bits() int {
 	return g.bitCount
 }
 
+// Append concatenates another GolombRice bit-stream onto this one.
+func (g *GolombRice) Append(other *GolombRice) {
+	if other.bitCount == 0 {
+		return
+	}
+	shift := g.bitCount & 63
+	targetSize := (g.bitCount + other.bitCount + 63) / 64
+	for len(g.data) < targetSize {
+		g.data = append(g.data, 0)
+	}
+	appendPtr := g.bitCount / 64
+	nWords := (other.bitCount + 63) / 64
+	if shift == 0 {
+		copy(g.data[appendPtr:], other.data[:nWords])
+	} else {
+		for i, w := range other.data[:nWords] {
+			g.data[appendPtr+i] |= w << shift
+			if appendPtr+i+1 < len(g.data) {
+				g.data[appendPtr+i+1] |= w >> (64 - shift)
+			}
+		}
+	}
+	g.bitCount += other.bitCount
+}
+
 func (g *GolombRiceReader) ReadReset(bitPos, unaryOffset int) {
 	g.currFixedOffset = bitPos
 	unaryPos := bitPos + unaryOffset