feat: add billing expression system documentation and enhance tiered billing logic

- Introduced a new rule for the Billing Expression System, emphasizing the importance of reading `pkg/billingexpr/expr.md` for dynamic billing.
- Updated the billing expression logic to support new variables and improved handling of image and audio tokens.
- Enhanced the tiered billing functionality with versioning support for expressions and refined quota calculations.
- Added tests to validate the new billing expression features and ensure correctness in pricing calculations.

pkg/billingexpr/types.go

@@ -20,6 +20,7 @@ type TokenParams struct {
 	CC   float64 // cache creation tokens (5-min TTL for Claude, generic for others)
 	CC1h float64 // cache creation tokens — 1-hour TTL (Claude only)
 	Img  float64 // image input tokens
+	ImgO float64 // image output tokens
 	AI   float64 // audio input tokens
 	AO   float64 // audio output tokens
 }
@@ -46,6 +47,7 @@ type BillingSnapshot struct {
 	EstimatedQuotaAfterGroup  int     `json:"estimated_quota_after_group"`
 	EstimatedTier             string  `json:"estimated_tier"`
 	QuotaPerUnit              float64 `json:"quota_per_unit"`
+	ExprVersion               int     `json:"expr_version"`
 }
 
 // TieredResult holds everything needed after running tiered settlement.