Sky/Sea Select: ?seat-aware so a CARTE owner drives all 6 seats — TDD

A Carte Blanche gamer owns all 6 seats but could only cast sky / draw sea for ONE:
the sky/sea state IS per-seat (Character.seat), but the code resolved to the fixed
canonical (PC-first) seat, ignoring the ?seat switched to via the GATE VIEW pos-
circles — so the tray switched but the sky wheel / sea spread stuck on the
canonical seat + saves wrote back to it. Mirrors Sig Select's existing ?seat path.

- generalize `_acting_sig_seat` → `_acting_seat` (logic is sig-agnostic; 3 callers)
- `_role_select_context` SKY_SELECT branch keys off `selected_seat` (the ?seat-aware
  seat, already computed) instead of `_canonical_seat`: user_polarity,
  confirmed_char, user_seat_role, my_tray_sig, saved_by_position, saved_sea_spread,
  sea_default_spread, hand_complete, sea_back_image_url
- sky_save / sky_delete / sea_save / sea_delete / sea_deck resolve the acting seat
  via `_acting_seat(…, request.GET.get("seat"))`; sea_partial threads seat_param
- the sky + sea felts carry `?seat={{ current_slot }}` on their save/delete/deck/
  sea_partial action URLs so the POSTs target the switched-to seat
- single-seat flow unchanged (no ?seat → canonical fallback)
- ITs: CARTE owner — ?seat switches the displayed spread; sea_save/sky_save target
  the switched seat leaving the canonical seat's Character intact; felt URLs carry
  ?seat. 949 epic+gameboard ITs green.

; FLIP tint tweak (parallel edit): drop the polarity border, bump the flipped-back
overlay --quiUser/--terUser to 0.6 alpha

[[project-sig-select-seat-switch-open-problems]] [[project-deck-segment-model]]
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -11,9 +11,9 @@
 <div class="sky-page sky-page--room"
      id="id_sky_overlay"
      data-preview-url="{% url 'epic:sky_preview' room.id %}"
-     data-save-url="{% url 'epic:sky_save' room.id %}"
-     data-delete-url="{% url 'epic:sky_delete' room.id %}"
-     data-sea-partial-url="{% url 'epic:sea_partial' room.id %}"
+     data-save-url="{% url 'epic:sky_save' room.id %}{% if current_slot %}?seat={{ current_slot }}{% endif %}"
+     data-delete-url="{% url 'epic:sky_delete' room.id %}{% if current_slot %}?seat={{ current_slot }}{% endif %}"
+     data-sea-partial-url="{% url 'epic:sea_partial' room.id %}{% if current_slot %}?seat={{ current_slot }}{% endif %}"
      data-user-seat-role="{{ user_seat_role }}">
 
     <div class="sky-modal-body">