fix(imagery): require explicit Data-Present proof before decoding thumbnails

Rebased onto current main with review follow-ups folded in.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

CHANGELOG.md

@@ -33,6 +33,8 @@
 
 - Director timeline seeks cross the dateline along the short longitude arc.
 
+- Recent Imagery thumbnails require explicit presence evidence before reporting available pixels.
+
 - Treat explicitly current, complete empty vessel coverage as a successful update,
   clearing obsolete contacts while retaining stale or incomplete snapshots.