refactor(verbose): replace the global flag and veprintln! with log

Diagnostics now go through the log facade: the library no longer
decides where output goes, and consumers plug in their own sink (the
CLI installs a stderr logger printing warnings always and ecr debug
diagnostics under -v; dependency records stay filtered out).  The
kernel 'quiet' flag moves from the global verbosity flag to an
explicit QemuConfig field.

Cargo.lock

@@ -307,6 +307,7 @@ dependencies = [
  "flate2",
  "futures-util",
  "libc",
+ "log",
  "nix",
  "reqwest",
  "serde",
@@ -329,6 +330,7 @@ dependencies = [
  "clap",
  "ecr",
  "indicatif",
+ "log",
 ]
 
 [[package]]