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- C4: Propagate request context through semaphores and upstream calls - C5: Make client rate limiter cleanup goroutine idempotent via sync.Once - P1: Streaming response path using io.Copy instead of full materialization - P2: Gate promotion goroutines to files >= 64KiB - P3: Reduce global lock contention during eviction - P4: Demote hot-path logging and compact metrics output - R1: Add upstream URL scheme/host validation - R4: Add cache file format version + tolerant deserialization - Makefile: Make build-snapshot-single practical by using -short tests - Config: Add GetDefaultConfig + Validate for better testability and R1 coverage All changes follow the "smallest safe diff" principle from the review. Safe test suite now builds and runs cleanly via make build-snapshot-single. Ref: docs/reviews/steamcache2-production-hardening-review-2026-05-26.md