The opening is a script now, not a slice in the wiring

bootOpening built its actions in boot.go and appended the finale by hand,
which made the one thing a player sees first the only content not
authored as content. inkwell's OnStart takes a script name now, so the
opening moves to script.opening.go and boot names it.

The -finale flag becomes OnFinale, the engine's hook for a closing script
queued after the start one, so the branch is a hook rather than a slice
append. bootOpening's start parameter had been dead for a while; it goes
with the function.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -350,8 +350,8 @@ Also: the inkwell README gives the module path as
   hides them; the deck wanted them struck through but visible, so the list
   becomes a memory of what you already tried. Needs a thin override over
   `DialogBox`.
-- **Only beat 3 exists.** The alley is a vertical slice; `bootOpening` sets the
-  police-tip flag that beat 2 will eventually set.
+- **Only beat 3 exists.** The alley is a vertical slice; the opening script sets
+  the police-tip flag that beat 2 will eventually set.
 - **The render has only been inspected once, by the author of this repo.**
   Geometry is derived from one font cell and one screen size, but this
   environment cannot take a screenshot (both synthetic keystrokes and screen