Warn when a usr1/usr2 restart leaves a config change unapplied

[facundofarias]

Problem

Supervisor#restart reloads the configuration before restarting anything, and Config#reload clears @options, @local_options and @environment_variables — so procodile restart genuinely does pick up Procfile/Procfile.options/Procfile.local changes.

Whether a given process sees them depends entirely on restart_mode:

restart_mode	behaviour	new environment applied?
start-term, term-start	spawns a replacement	yes
usr1, usr2	::Process.kill(mode, @pid)	no

usr1/usr2 signal the running process rather than replacing it. The environment is only ever applied at Process.spawn, so such a process keeps the environment it was born with indefinitely — while Procodile reports the restart as successful.

This is genuinely hard to diagnose from outside: the config on disk is correct, status looks healthy, and only the application's behaviour disagrees. We lost time to exactly this — an environment variable added to Procfile.local, procodile restart run, and the value never reached the process.

Change

Record the environment and command each instance was actually spawned with. After signalling a usr1/usr2 process, log a warning if either has since changed:

Warning: environment changed since this process started, but usr1 signals the
existing process rather than replacing it. Stop and start this process for the
change to take effect.

Notes

• Fires only when something actually changed, so routine restarts stay quiet.
• Instances this supervisor never spawned record nothing and are skipped, so there are no false warnings after a supervisor restart.
• No behaviour change — this is diagnostics only. Signalling semantics are untouched.

Specs in spec/specs/instance_spec.rb cover unchanged, environment-changed, variable-added, command-changed, both-changed, and never-spawned. Full suite green (40 examples).