refactor(sync): move impl/loops out of mod.rs into sibling files

mod.rs files must contain only module declarations. Two violated this:

- sync_delegate/mod.rs held the entire ~570-line `SyncDelegate` impl. A
  single trait impl cannot be split across multiple `impl` blocks, so the
  whole block moves to sync_delegate/delegate_impl.rs; mod.rs is now just
  `mod` declarations. Relative paths adjusted for the deeper module.
- sync/transport/push/mod.rs held `delta_push_loop`/`ping_loop` (plus the
  stale-timeout const). These move to push/loops.rs, re-exported from
  mod.rs so `transport::push::{delta_push_loop, ping_loop}` still resolve.

Pure relocation — no behavior change. Verified: compiles, clippy clean,
transport dispatch tests (announce-before-delta ordering) + handshake
E2E all green.

Author: farhan-syah