R2-8.2: replace tokio full with granular features in zeph-core

[bug-ops]

Epic: #282 | Phase: 1 | Priority: P2 | Size: S

Problem

zeph-core uses tokio = { features = ["full"] } compiling unnecessary features (io-util, signal, fs, net, process).

Solution

Replace with specific features: ["rt-multi-thread", "sync", "time", "macros"].