gofmt: bring internal/ and cmd/ back to gofmt -l clean

Mechanical `gofmt -w ./internal ./cmd`. Mostly struct-field realignment that
had drifted, plus a few trailing-newline fixes. No behaviour change — gofmt is
semantics-preserving, and build/vet/test are green either side.

Split out from the code-review fixes that follow so those stay reviewable
instead of hiding inside a wall of realignment.

internal/plugin/zone_graph_nav.go

@@ -427,4 +427,3 @@ func resolveForkChoice(pf *pendingFork, choice int) (pendingChoice, error) {
 	}
 	return c, nil
 }
-