Add core components for request coalescing and service management

- Introduced coalescing logic in `coalescing.go` to handle concurrent identical upstream fetches, including a state machine and response buffering for improved performance.
- Implemented a new cache file format in `format.go`, supporting serialization and deserialization of HTTP responses, along with range request handling.
- Developed an HTTP handler in `handler.go` to manage requests, including special endpoint handling and metrics reporting.
- Added rate limiting functionality in `ratelimit.go` to control per-client and global request rates, enhancing security and performance.
- Created service management capabilities in `service.go` to define and manage cacheable services, including user-agent detection.
- Updated tests in `steamcache_test.go` to cover new functionalities, ensuring robustness and reliability across the codebase.

vfs/memory/memory.go

@@ -319,7 +319,9 @@ func (m *MemoryFS) EvictLRU(bytesNeeded uint) uint {
 			break
 		}
 		fi := elem.Value.(*types.FileInfo)
-		toEvict = append(toEvict, fi.Key)
+		key := fi.Key
+		m.LRU.Remove(key) // actually remove during collection so Back() advances to distinct items
+		toEvict = append(toEvict, key)
 		cur -= fi.Size // local estimate; real size updated in W phase
 	}
 	m.mu.Unlock()