Two copies of natsHeaderCarrier existed (leaf/agent/nats.go and leaf/agent/notify/pubsub.go), both carrying the same latent case-sensitive Get bug that was fixed in Task 6. Extract the type to leaf/agent/internal/natshdr so the next bug can be fixed once.

While consolidating, drop the 4-tier fallback in Get in favor of a single EqualFold scan. Header maps are tiny (typically 3 entries) and tiers 1-3 were subsumed by tier 4 anyway. The new implementation is demonstrably correct and easier to read. check-in: b53032d01f user: [email protected] tags: trunk, leaf/v0.0.2