Convert repository into Cargo workspace with modular crates
- Split code into workspace members: miner-cli, miner-service, pow-core,
engine-cpu, engine-gpu-cuda, engine-gpu-opencl, metrics - Moved QPoW
math logic into local pow-core crate with compatibility API and
incremental helpers - Defined MinerEngine trait and baseline CPU engine
in engine-cpu crate - Rewrote service layer in miner-service: job
management, HTTP API, engine orchestration - Added CLI binary crate
miner-cli with flags: --port, --num-cores, --metrics-port, --engine -
Implemented runtime toggle for Prometheus metrics exporter enabled only
when --metrics-port provided - Deprecated root src/lib.rs and
src/main.rs with notices pointing to new CLI/workspace entrypoints
rob thijssen
