Tighten DHCP and PXE Visibility

[chet]

The DHCP/PXE stack contains 242 compiler-proven overbroad declarations: 184 in dhcp, 35 in dhcp-server, and 23 in pxe. Eighty of the DHCP findings live in integration-test support, where public declarations are only serving modules inside the test crate.

What this involves

• Narrow DHCP production modules, DHCP integration-test helpers under tests/common/**, DHCP Server internals, and PXE implementation code according to their actual callers.
• Keep gRPC/generated modules at the VSC-01 boundary and preserve any feature-gated cross-crate test support.
• Remove or correctly gate newly unused helpers; do not add dead-code allowances to replace accidental pub.
• Run targeted tests for all three crates plus the full lint and visibility commands with no finding left in scope.
• Include the campaign sentence from the Epic in both the commit message and PR description.

Part of #4545.