Implement Banded LCS calculation with scalar and SIMD support

Introduces the BandedLCS function for efficient LCS scoring, supporting both scalar and SIMD implementations. Adds comprehensive unit tests, benchmarking utilities, and error handling for sequence constraints.

pkg/obialign/lcsband_nosimd.go

@@ -0,0 +1,13 @@
+//go:build !goexperiment.simd
+
+package obialign
+
+const haveSIMD = false
+
+// encodeBases and bandedLCSSIMD are never called without GOEXPERIMENT=simd
+// (haveSIMD is false); they exist to keep the dispatcher uniform.
+func encodeBases(ws *bandedLCSWorkspace, a, b []byte) {}
+
+func bandedLCSSIMD(lA, lB, kmin, kmax int, ws *bandedLCSWorkspace) uint32 {
+	return 0
+}