feat(adapter): add passthrough adapter for unchanged vectors

pkg/adapter/adapter.go

@@ -41,6 +41,17 @@ type Adapter interface {
 	TargetDim() int
 }
 
+// passthroughAdapter returns the input vector unchanged.
+type passthroughAdapter struct{}
+
+func (passthroughAdapter) Adapt(vec []float32) ([]float32, error) { return vec, nil }
+func (passthroughAdapter) SourceDim() int                         { return 0 }
+func (passthroughAdapter) TargetDim() int                         { return 0 }
+
+// NewPassthrough returns an Adapter that returns vectors unchanged.
+// Use when no dimension adaptation is needed.
+func NewPassthrough() Adapter { return passthroughAdapter{} }
+
 // NewTruncate returns a TruncateAdapter for Matryoshka-style or simple truncation/padding.
 // t controls which end is dropped when downscaling; p controls which end is padded when upscaling.
 func NewTruncate(sourceDim, targetDim int, t TruncateMode, p PadMode) (Adapter, error) {