Migrate remaining services to the shared logfmt logging layer

[adnandnv]

Summary

PR #2354 (issue #2049) added a structured component field to NICo's logfmt logging
layer, so logs can be filtered by the emitting component. A few services still don't
use the logfmt layer, so they carry no component and emit in a different format. There's
no good reason for services to diverge — they should move onto the shared logfmt
layer for consistent, filterable structured logging.

Services to migrate

• nico-dns (carbide-dns) — currently fmt::layer().json() (JSON). Small swap:
  already on tracing_subscriber, just replace the fmt layer with logfmt::layer().
• nico-ssh-console (carbide-ssh-console) — currently
  fmt::Layer::default().compact(). Small swap, same as above.
• nico-pxe (carbide-pxe) — larger. It hand-rolls logging with
  println!/eprintln! and uses no tracing at all, so this means first adopting
  tracing (convert the println!/eprintln! call sites to tracing::{info,warn,error}!)
  and then initializing the logfmt layer.

Each migrated service gets a component default via
logfmt::layer().with_event_fields([logfmt::EventField::with_default("component", "nico-…")]).

Out of scope

• nico-dpu-otel-agent — a custom build of the upstream OpenTelemetry Collector with
  minimal plugin extensions; don't modify its logging.
• CLI / dev / mock / test / utility binaries (admin-cli, bmc-explorer-cli, bmc-mock,
  mockdpa, machine-a-tron, mqttea-example, dpf api-harness, libmlx mlxconfig-*) — not
  centralized-logging services.

Why

• One consistent, filterable structured-log format across NICo's Rust services in
  centralized logging.
• Each service gets a component value, so its lines can be selected directly instead of
  relying on log-stream selectors.

Acceptance criteria

• Each in-scope service initializes the logfmt layer at startup and emits
  level=… component=nico-… … logfmt lines to stdout, with the correct per-binary default.
• docs/observability/logging.md "Coverage" section updated as each lands: move the
  migrated service out of the "do not use logfmt" list; keep nico-dpu-otel-agent listed
  with its rationale.

References

• Issue feat: carbide-api: Add a component field to log message metadata #2049 (the component field); PR feat(logging): add a component field to log metadata #2354.
• Reviewer note: feat(logging): add a component field to log metadata #2354 (comment)