refactor cmd: thin main, testable run(io.Writer), AllStrategies in engine

The strategy list moves into the engine (AllStrategies), where the API
and frontend will read it too: one source of truth, per the handoff.
main shrinks to the standard Go shell pattern: all work happens in
run(out io.Writer) error; main only maps the error to stderr and the
exit code. Writing to an interface instead of stdout is what lets
main_test.go capture output in a bytes.Buffer.
errcheck flagged every unchecked Fprintf, so formatting became pure
Sprintf string building with one checked write at the end: nicer than
discarding four errors with '_, _ ='.

internal/engine/scenario.go

@@ -11,6 +11,12 @@ const (
 	StrategyMostConnected Strategy = "most-connected"
 )
 
+// AllStrategies lists every strategy in display order. The CLI, and later
+// the API and frontend, read this one list; nothing redefines it.
+func AllStrategies() []Strategy {
+	return []Strategy{StrategyNone, StrategyRandom, StrategyMostConnected}
+}
+
 // Config fully describes one simulation world. It is the single source of
 // truth for parameters: API and frontend types will be generated from the
 // structs in this file, never redefined by hand. Identical configs produce