[proxy] split prompt-cache cost out of the metered total

Pricing now returns the cache portion of the request cost next to the
total (cache read + creation for Anthropic-shape providers, the
discounted cached subset for OpenAI). cost_meter emits it as
cost.usd_cache, and the usage-only access-log strip keeps the cache
token counts and cache cost so management can persist them.

proxy/internal/llm/pricing/pricing.go

@@ -128,6 +128,20 @@ type Table struct {
 //   - Other providers: cached and cacheCreation are ignored; cost is
 //     inTokens*InputPer1K + outTokens*OutputPer1K.
 func (t *Table) Cost(provider, model string, inTokens, outTokens, cachedInput, cacheCreation int64) (float64, bool) {
+	c, ok := t.Costs(provider, model, inTokens, outTokens, cachedInput, cacheCreation)
+	return c.TotalUSD, ok
+}
+
+// Costs is a per-request cost split. CacheUSD is the portion of TotalUSD billed for
+// prompt-cache buckets and is always <= TotalUSD.
+type Costs struct {
+	TotalUSD float64
+	CacheUSD float64
+}
+
+// Costs returns the estimated USD cost split for the given token counts, with
+// the same semantics as Cost.
+func (t *Table) Costs(provider, model string, inTokens, outTokens, cachedInput, cacheCreation int64) (Costs, bool) {
 	// Clamp negatives to zero before any pricing math so a malformed
 	// upstream count can never produce a negative cost.
 	if inTokens < 0 {
@@ -143,15 +157,15 @@ func (t *Table) Cost(provider, model string, inTokens, outTokens, cachedInput, c
 		cacheCreation = 0
 	}
 	if t == nil {
-		return 0, false
+		return Costs{}, false
 	}
 	byModel, ok := t.entries[provider]
 	if !ok {
-		return 0, false
+		return Costs{}, false
 	}
 	entry, ok := byModel[model]
 	if !ok {
-		return 0, false
+		return Costs{}, false
 	}
 	output := (float64(outTokens) / 1000.0) * entry.OutputPer1K
 	switch provider {
@@ -168,7 +182,7 @@ func (t *Table) Cost(provider, model string, inTokens, outTokens, cachedInput, c
 		}
 		nonCached := float64(inTokens-clamped) / 1000.0 * entry.InputPer1K
 		cached := float64(clamped) / 1000.0 * cachedRate
-		return nonCached + cached + output, true
+		return Costs{TotalUSD: nonCached + cached + output, CacheUSD: cached}, true
 	case "anthropic", "bedrock":
 		// Bedrock-Anthropic returns the same additive cache buckets as
 		// first-party Anthropic; non-Anthropic Bedrock models simply report
@@ -184,10 +198,10 @@ func (t *Table) Cost(provider, model string, inTokens, outTokens, cachedInput, c
 		input := float64(inTokens) / 1000.0 * entry.InputPer1K
 		read := float64(cachedInput) / 1000.0 * readRate
 		create := float64(cacheCreation) / 1000.0 * createRate
-		return input + read + create + output, true
+		return Costs{TotalUSD: input + read + create + output, CacheUSD: read + create}, true
 	default:
 		input := float64(inTokens) / 1000.0 * entry.InputPer1K
-		return input + output, true
+		return Costs{TotalUSD: input + output}, true
 	}
 }