Adds a FiberDriver to the Concurrency module that provides real
cooperative async I/O using the Revolt event loop and amphp. Unlike
ProcessDriver (child processes) or SyncDriver (sequential), tasks
genuinely interleave on I/O suspension within a single process.
Tested with amphp/http-client (3x1s HTTP in 1.96s), amphp/mysql
(3x100ms queries in 185ms), and amphp/redis (100 keys in 13.5ms).
No background process needed — the event loop runs inline within run().
Shared memory, no serialization overhead, works in web and CLI.