sea stage: gate the FLIP-back 0.3 polarity tint to the cloned dubbodeck (Sea Select); my_sea/visit monodeck backs render un-tinted

- the `.sea-stage--gravity/levity .sea-stage-card.is-flipped-to-back::after` 0.3 fill differentiates the two halves of a monodeck CLONED into a two-toned dubbodeck — only meaningful in Sea Select. my_sea / my_sea_visit draw a single monodeck that is never cloned, so the tint was just noise on their FLIP'd card-back
- the shared `_sea_stage.html` now takes a `sea_stage_dubbo` flag; only `_sea_overlay.html` (the gameroom Sea Select felt) passes it → `.sea-stage--dubbo` on #id_sea_stage. The two tint rules gate on `.sea-stage--dubbo.sea-stage--gravity/levity`; non-dubbo stages fall through to the empty, fill-less base `::after` (no visible overlay)

[[project-deck-segment-model]]

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/templates/apps/gameboard/_partials/_sea_overlay.html

@@ -146,7 +146,10 @@ via epic:sea_save. `?seat` threads the CARTE-selected seat onto the action URLs.
     </div>
 
     {# Sea stage — portaled big-card viewer (position:fixed, escapes the snap). #}
-    {% include "apps/gameboard/_partials/_sea_stage.html" %}
+    {# `sea_stage_dubbo` flags the gameroom's cloned two-toned dubbodeck so the #}
+    {# FLIP-back polarity tint applies HERE only (my_sea/visit draw a monodeck  #}
+    {# that is never cloned — no tint there). See _card-deck.scss.              #}
+    {% include "apps/gameboard/_partials/_sea_stage.html" with sea_stage_dubbo=True %}
 </div>
 </div>