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/_sea_overlay.html

@@ -14,9 +14,9 @@ Each draw persists onto the seat's Character.celtic_cross via epic:sea_save.
 {% endcomment %}
 <div class="sea-page sea-page--room" id="id_sea_page">
 <div class="my-sea-picker{% if hand_complete %} my-sea-picker--locked{% endif %}" id="id_sea_overlay"
-     data-sea-deck-url="{% url 'epic:sea_deck' room.id %}"
-     data-sea-save-url="{% url 'epic:sea_save' room.id %}"
-     data-sea-delete-url="{% url 'epic:sea_delete' room.id %}"
+     data-sea-deck-url="{% url 'epic:sea_deck' room.id %}{% if current_slot %}?seat={{ current_slot }}{% endif %}"
+     data-sea-save-url="{% url 'epic:sea_save' room.id %}{% if current_slot %}?seat={{ current_slot }}{% endif %}"
+     data-sea-delete-url="{% url 'epic:sea_delete' room.id %}{% if current_slot %}?seat={{ current_slot }}{% endif %}"
      data-sea-user-polarity="{{ user_polarity }}">
 
     {# ── Felt cross — the REAL deal target (.my-sea-cross). Sig pins core; the #}