Add Non-Interactive Launch Heartbeats

[drown0315]

Parent

#90

What to build

Add periodic heartbeat output for non-interactive Target App Launch Progress.
When app launch takes a long time in CI or redirected output, Flutter Pilot
should print deterministic status lines at a low cadence so logs show the
command is still active without being flooded.

The initial cadence should be ten seconds. Heartbeats should use the same launch
status wording as plain Target App Launch Progress and should stop once launch
succeeds or fails.

Acceptance criteria

• Non-interactive launch progress prints heartbeat status while waiting for the Runtime Target.
• The initial heartbeat cadence is ten seconds.
• Heartbeat lines include elapsed launch time.
• Heartbeat output stops after launch success or launch failure.
• Heartbeat output is written to stderr.
• Interactive terminal output does not use the non-interactive heartbeat line behavior.
• Tests use a fake clock or injected ticker to verify heartbeat cadence without slow real-time waits.

Blocked by

• Add Plain Target App Launch Progress #91