Record the spent-project reversal as ADR 0007

The project file existed to persist state, so a future reader finding
the exported flag would otherwise re-litigate it. The ADR states plainly
that this reverses an earlier decision, what the original reasoning was,
and what changed in use.

ADR 0002 deferred the SVG renderer partly because re-export made it free
to add later. That no longer holds, so its 'not stranded' bullet is
struck through and pointed at ADR 0007 rather than left standing to
mislead whoever revisits the SVG question.

docs/adr/0002-raster-only-svg-renderer-deferred.md

@@ -54,8 +54,11 @@ degraded fallback.
 
 - The geometry model stays **renderer-agnostic**, in normalised page coordinates,
   so adding the SVG renderer later is an output stage rather than a redesign.
-- **Re-export from the project file** regenerates every song's bundle without
+- ~~**Re-export from the project file** regenerates every song's bundle without
   repeating human work, so songs cut before the SVG renderer exists are not
-  stranded.
+  stranded.~~ **No longer true** — see
+  [ADR 0007](0007-a-project-is-spent-once-exported.md). A project is spent once
+  its song has been exported, so songs cut before the SVG renderer ships stay
+  WebP unless they are cut again by hand.
 - noteman needs no SVG support (`image/svg+xml`, `.svg` content type, CSP header
   on SVG responses) until the renderer ships.