fst: move App interface declaration

Signed-off-by: Ophestra <cat@gensokyo.uk>

fst/app.go

@@ -0,0 +1,33 @@
+package fst
+
+import "context"
+
+type App interface {
+	// ID returns a copy of App's unique ID.
+	ID() ID
+	// Run sets up the system and runs the App.
+	Run(ctx context.Context, rs *RunState) error
+
+	Seal(config *Config) error
+	String() string
+}
+
+// RunState stores the outcome of a call to [App.Run].
+type RunState struct {
+	// Start is true if fsu is successfully started.
+	Start bool
+	// ExitCode is the value returned by shim.
+	ExitCode int
+	// WaitErr is error returned by the underlying wait syscall.
+	WaitErr error
+}
+
+// Paths contains environment-dependent paths used by fortify.
+type Paths struct {
+	// path to shared directory e.g. /tmp/fortify.%d
+	SharePath string `json:"share_path"`
+	// XDG_RUNTIME_DIR value e.g. /run/user/%d
+	RuntimePath string `json:"runtime_path"`
+	// application runtime directory e.g. /run/user/%d/fortify
+	RunDirPath string `json:"run_dir_path"`
+}