feat(memory): M4 记忆完善（三类作用域访问规则+项目索引分层+LLM异步提取+衰减溯源+向量接入，兼容既有57用例）

internal/runtime/memory/decay.go

@@ -0,0 +1,23 @@
+package memory
+
+import (
+	"math"
+	"time"
+)
+
+// decayFactor computes the decay multiplier for a single item: the stored
+// decay_weight scaled by a time-decay that halves every recencyHalfLife days
+// (design §2 衰减: 久未使用的记忆降权). A freshly touched item keeps its full
+// decay_weight; an item unused for N half-lives is scaled to
+// decay_weight * 0.5^N, so it ranks progressively lower unless it stays relevant.
+//
+// age is measured from UpdatedAt; a future timestamp (clock skew) is clamped to
+// zero, so a just-recorded item is never scored above its base weight.
+func (r *Retriever) decayFactor(it MemoryItem, now time.Time) float64 {
+	age := now.Sub(it.UpdatedAt)
+	if age < 0 {
+		age = 0
+	}
+	halfLives := float64(age) / float64(r.recencyHalfLife)
+	return it.DecayWeight * math.Pow(0.5, halfLives)
+}