mlxrunner: Cache.Update takes ForwardBatch and returns KVHistory

Signature changes from Update(k, v) to Update(batch, k, v) returning
(k, v, KVHistory). KVCache returns a real page table mapping positions
to buffer slots. RecurrentCache returns empty KVHistory from Update.

Replace Cache.Offset() with Offsets() returning per-sequence offsets.
Add KVHistory type to mlx package.

x/mlxrunner/mlx/sdpa.go

@@ -0,0 +1,13 @@
+package mlx
+
+// KVHistory carries sequence metadata alongside K/V buffers for SDPA.
+// Page table and seq lens travel together — SDPA always needs both.
+type KVHistory struct {
+	// PageTable maps (seqIdx, position) → slot index in the K/V buffer.
+	// Shape: [numSeqs, maxSeqLen], int32. Unused entries are 0.
+	PageTable *Array
+
+	// SeqLens is the history length per sequence (number of valid
+	// entries in each row of PageTable).
+	SeqLens []int
+}