tray: Tray.placeSig analogue of placeCard for SIG SELECT exit; rename arc-in → fade-in — TDD

After all 3 gamers in a polarity room confirm TAKE SIG and the 12s countdown
expires, sig-select.js's room:polarity_room_done handler now plays the same
tray-open / fade-in / tray-close sequence the role-select uses, then
dismisses the sig overlay & shows the waiting msg ("Gravity settling…" /
"Levity appraising…") on Tray.placeSig's completion callback. Visual order:
sig stage → tray slides in → sig fades into the second tray cell → tray
slides out → table hex w. waiting msg. Cross-polarity events (other room
finishing while we're still in our overlay) are no-op as before.

- tray.js: new Tray.placeSig(sourceEl, onComplete). Mutates the SECOND
  .tray-cell in place (sig slot), copies aria-label / data-energies /
  data-operations / corner-rank + suit-icon markup from the source
  .sig-stage-card, then runs the shared open → fade-in → close sequence.
  Extracted _runFadeInSequence helper so placeCard + placeSig share the
  same animation glue. reset() now also clears .tray-sig-card from cells.

- _tray.scss: .tray-sig-card.fade-in > .sig-stage-card animates via the
  existing tray-role-fade-in keyframes.

- sig-select.js polarity_room_done handler: Tray.placeSig(stageCard,
  _settle); _settle runs the existing _dismissSigOverlay + _showWaitingMsg.
  Falls back to immediate dismiss when Tray is undefined (test environments
  without the tray).

- arc-in → fade-in rename across tray.js, role-select.js, _tray.scss
  (incl. @keyframes tray-role-arc-in → tray-role-fade-in), TraySpec.js
  spec descriptions + assertions, & test_room_role_select.py docstrings.
  The original "arc-in" name suggested a curved-path animation; the actual
  behaviour is a 1s opacity fade, so fade-in is the accurate label.

- TraySpec: 10 new placeSig specs mirroring placeCard (second-cell mutation,
  data + markup copy, tabIndex, fade-in class, animationend-triggered close,
  onComplete callback, landscape parity, reset cleanup).

- SigSelectSpec: 3 new specs (Tray.placeSig called w. stageCard on own
  polarity; not called on other polarity; overlay dismiss deferred to the
  Tray.placeSig completion callback).

344 specs / 4 pending green; RoleSelectTrayTest FT still green.

Code architected by Disco DeDisco <discodedisco@outlook.com>
Git commit message Co-Authored-By:
Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/static_src/scss/_tray.scss

@@ -123,7 +123,7 @@ $tray-bevel:    0.3rem;  // inner bevel ring; grid must sit inside this
 }
 
 // ─── Role card: scrawl fade-in ───────────────────────────────────────────────
-@keyframes tray-role-arc-in {
+@keyframes tray-role-fade-in {
     from { opacity: 0; }
     to   { opacity: 1; }
 }
@@ -162,8 +162,8 @@ $tray-bevel:    0.3rem;  // inner bevel ring; grid must sit inside this
     }
 
     // Cell stays static; only the scrawl image fades in.
-    &.arc-in img {
-        animation: tray-role-arc-in 1s ease forwards;
+    &.fade-in img {
+        animation: tray-role-fade-in 1s ease forwards;
     }
 }
 
@@ -191,6 +191,12 @@ $tray-bevel:    0.3rem;  // inner bevel ring; grid must sit inside this
     &.tt-active > .sig-stage-card {
         rotate: 7deg;
     }
+
+    // Fade-in mirrors .tray-role-card.fade-in img — the .sig-stage-card child
+    // fades from opacity 0 to 1 once placeSig() lands.
+    &.fade-in > .sig-stage-card {
+        animation: tray-role-fade-in 1s ease forwards;
+    }
 }
 
 @keyframes tray-wobble {