feat(chat): StreamerHub with ring buffer + cond + Done/Error/Cancel flows

internal/chat/pricing.go

@@ -0,0 +1,15 @@
+package chat
+
+import "strconv"
+
+// computeCost returns the cost in USD for a single LLM call.
+// Prices on LLMModel are expressed in USD per million tokens.
+func computeCost(m LLMModel, ptok, ctok, ttok int) float64 {
+	return (float64(ptok)*m.PromptPricePerMillionUSD +
+		float64(ctok)*m.CompletionPricePerMillionUSD +
+		float64(ttok)*m.ThinkingPricePerMillionUSD) / 1_000_000
+}
+
+// int64ToString converts an int64 to its decimal string representation.
+// Uses strconv.FormatInt rather than fmt.Sprintf for efficiency.
+func int64ToString(v int64) string { return strconv.FormatInt(v, 10) }