feat(control): instrument async control client entry points

[forkwright]

Summary

• Add tracing::debug! events at the four async public entry points on ControlClient (register, poll_registration, start_map_stream, recv_map_update) under the dictyon::control target.
• First narrow slice of dictyon library tracing under Phase 3 audit: missing PeersChangedPatch + Node fields, no zstd, transport duplication, no tracing, no map-stream integration test #20. Does not touch transport/wire/noise instrumentation, which still need a deliberate span/event-scheme pass.

Why

• crates/dictyon/examples/connect.rs already emits tracing::info!/warn! for the control flow, but no events fire from the library itself, so any consumer of dictyon::control::ControlClient has zero visibility into registration outcome, followup polling, map-stream initiation, or keepalive vs. non-keepalive map updates without re-wrapping every call site.
• Adding events at the four public async entry points gives that visibility without picking a span model that would constrain the larger transport/wire/noise tracing pass.

Test plan

• cargo fmt --all --check
• cargo check --workspace --all-targets
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets
• git diff --check

Refs #20