fix(diff): correct overlap detection and bounds in linear space Myers (#46)

* fix(diff): correct overlap detection and bounds in linear space Myers

This commit resolves two critical pathfinding bugs in the linear space
Myers algorithm (`findMiddleSnake`) that caused it to produce suboptimal
diffs or silently fail to explore valid search spaces.

1. Fix forward search overlap detection:
During the forward search overlap check (when `delta` is odd), the
algorithm incorrectly checked the reverse vector `vr` at `offset + (delta - k)`.
However, the reverse search already maps its furthest-reaching `x` bounds
to the same forward diagonal `k`. Checking the wrong index caused the
algorithm to miss the true optimal middle snake, leading to longer,
suboptimal edit scripts. This is fixed by correctly checking
`reverseIdx := offset + k`.

2. Fix vector sizing and offset for negative delta:
When sequence A is shorter than sequence B (`delta < 0`), the forward-diagonal
`k` in the reverse search can reach down to `-maxDiff + delta`. Because
`offset` was previously only adjusted for positive `delta`, calculating
`offset + k` would result in negative indices. This caused the reverse search
to silently trigger bounds checks and skip exploring half of its required
search grid. The `vectorSize` and `offset` are now correctly adjusted when
`delta < 0` to accommodate the full bounds.

* chore: add comments

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: KimNorgaard

diff/myers/myers.go

@@ -339,11 +339,13 @@ func findMiddleSnake(a, b []string, aStart, aEnd, bStart, bEnd int) snake { //re
 	// vr: Reverse search (from bottom-right)
 	// Size needs to accommodate k ranging from -maxDiff to +maxDiff.
 	vectorSize := 2*maxDiff + 1
+	offset := maxDiff
 	if delta > 0 {
 		vectorSize += delta // Ensure space for reverse search diagonals
+	} else if delta < 0 {
+		vectorSize -= delta // delta is negative, so this increases the size
+		offset -= delta     // delta is negative, so this increases the offset
 	}
-	// offset: Used to map diagonal k (which can be negative) to non-negative array indices.
-	offset := maxDiff
 
 	vf := make([]int, vectorSize)
 	vr := make([]int, vectorSize) // Stores relative x-coordinates within the n x m grid
@@ -448,7 +450,7 @@ func findMiddleSnake(a, b []string, aStart, aEnd, bStart, bEnd int) snake { //re
 			//  Overlap Check
 			// Overlap check
 			if delta%2 != 0 && k >= delta-(d-1) && k <= delta+(d-1) {
-				reverseIdx := offset + (delta - k)
+				reverseIdx := offset + k
 				if reverseIdx >= 0 && reverseIdx < vectorSize {
 					reverseX := vr[reverseIdx]
 					if reverseX >= 0 && x >= reverseX {