Bounded nearby-window line search must start from both gap ends

When you speed up diff-based line relocation by searching only the N lines nearest a mark's expected position in an unmatched gap, an offset from the gap's top breaks when more than N lines are inserted right above the mark. The fix is to search outward from both the top-relative target and the end-relative target (newGap.end - (oldGap.end - line)), interleaved by distance with a seen-set, keeping O(N) per mark. A regression check with N+44 lines inserted above an edited line catches it. A test with fewer than N lines inserted won't.