Migrate the dhcp crate from the log facade to structured tracing

[chet]

What

The dhcp crate is the codebase's lone log:: facade island: 47 of the workspace's 50 log:: macro calls live in crates/dhcp (the other 3 are in crates/rpc), while everything else uses tracing:: -- ~3,380 calls. So dhcp's logs don't carry structured fields into the fleet's logfmt pipeline the way tracing:: logs do, and they may not even reach it.

Investigate first

• WHY dhcp uses log:: -- most likely because it's built around dhcproto (its DHCP protocol dependency), which emits through the log facade, so dhcp matched it. Confirm.
• Whether dhcp-server (the binary consuming carbide-dhcp) installs a log->tracing bridge (LogTracer / tracing-log). If it does, dhcp's log:: records reach the fleet subscriber but as unstructured messages; if not, they may be dropped entirely -- a real observability gap.

Then migrate

• Convert dhcp's ~47 log::{warn,info,error,debug,trace}! sites (and rpc's 3) to tracing::, passing values as structured fields (error = %e, %address, ...) instead of interpolating them.
• Decide how to handle dhcproto's own log:: output (keep a bridge for the dependency's logs, or leave it).

Surfaced during #3469 review (CodeRabbit flagged dhcp's log::warn! at crates/dhcp/src/lib.rs); deferred so it can be done crate-wide rather than piecemeal.