docs: scrub dangling references to deleted roadmap docs

The ROADMAP/ROADMAP-PINCHFLAT docs were removed; this drops the now-dead
"see ROADMAP-PINCHFLAT.md" breadcrumbs (and one "the roadmap" mention)
from source comments, preserving the surrounding Pinchflat-style context.
Comment-only; typecheck (node+web) green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/preload/index.ts

@@ -194,7 +194,7 @@ const api = {
     return () => ipcRenderer.removeListener(IpcChannels.externalUrl, listener)
   },
 
-  // --- Media-manager sources (Pinchflat-style; see ROADMAP-PINCHFLAT.md) ---
+  // --- Media-manager sources (Pinchflat-style) ---
 
   listSources: (): Promise<Source[]> => ipcRenderer.invoke(IpcChannels.sourcesList),