burger z-index drop 318→314 to match .gear-btn; FT base dismiss_brief_if_present helper — TDD

Follow-up to 3ca986f. Lands the FT fixes the burger sprint surfaced + tightens the burger's z-stack so the existing kit_btn / bud_btn / bud_panel / dialog naturally cover it on overlap (vs. the explicit opacity-fade rules the first iteration tried).

## Burger z-index drop

`static_src/scss/_burger.scss`:
- `#id_burger_btn` z 318 → 314 (matches the page-level `.gear-btn` z). Below kit_btn + bud_btn (318), bud_panel (317), kit_bag_dialog (316) — so every overlapping surface visually covers the burger when it appears. The earlier `html.bud-open #id_burger_btn { opacity: 0 }` + `html:has(#id_kit_bag_dialog[open]) #id_burger_btn { opacity: 0 }` rules are now redundant + deleted.
- `#id_burger_fan` z 317 → 313 (stays just below burger so burger remains clickable when fan is open).

`static_src/scss/_game-kit.scss`:
- `#id_kit_bag_dialog` z 319 → 316 (reverts an earlier iteration that bumped it above burger). 316 keeps the dialog BELOW kit_btn + bud_btn so those stay visible + clickable when dialog opens — user re-clicks kit_btn to close. Resolves "kit btn disappears when dialog open" reported on iPad portrait.

`static_src/scss/_bud.scss`:
- `html.bud-open #id_kit_btn { opacity: 0 }` now wrapped in `@media (orientation: portrait)`. In landscape kit_btn lives at the TOP of the right sidebar + bud_panel sits at the BOTTOM — no visual conflict, kit stays visible.

## FT base — dismiss_brief_if_present()

`functional_tests/base.py`:
- New helper on both `FunctionalTest` + `ChannelsFunctionalTest`:
  ```python
  def dismiss_brief_if_present(self, banner_selector=".note-banner", browser=None):
  ```
- Removes any matching Brief banner from the DOM via `execute_script`. No-op if absent. Default selector matches every Brief shape; pass a specific selector to target one kind. DOM-removal rather than NVM-click bypasses the dismiss_url POST flow that some Briefs (FREE/PAID DRAW) wire up — use this when the test cares about the page state AFTER a Brief, not the dismissal mechanics.

## FT — test_trinket_coin_on_a_string.py

Two methods (`test_coin_deposit_unequips_from_kit_bag_and_fills_one_slot`, `test_coin_in_use_game_kit_shows_room_attribution_and_btn_disabled`) updated:

1. `self.dismiss_brief_if_present()` right after `wait_for(id_game_kit)`. `coin@test.io` is created fresh w/o a significator, so the `_my_sea_sign_gate_brief.html` auto-spawns on `/gameboard/` + intercepts the create-game btn click. Dismissing the banner clears the runway.

2. `self.wait_for(... dialog.rect["width"] > 50 ...)` after `kit_btn.click()` + before interacting w. tokens inside the dialog. The landscape kit_bag_dialog animates `max-width 0 → 5rem` over 0.25s; the existing wait_for(find_token).click() found the COIN .token in the DOM immediately + raced the animation — Selenium's scrollIntoView fails on a 0-width container. Waiting for the rect to widen past 50px (≈ 3rem) confirms layout has rendered.

## Verification

- All 4 previously-failing FTs from the burger sprint re-run green:
  - CarteBlanche.test_carte_blanche_equip_and_multi_slot_gatekeeper (was flaking; passed on retry — pre-existing tooltip-population race, not in scope)
  - CoinOnAString.test_coin_deposit_unequips_from_kit_bag_and_fills_one_slot
  - CoinOnAString.test_coin_in_use_game_kit_shows_room_attribution_and_btn_disabled
  - GatekeeperTest.test_second_gamer_drops_token_into_open_slot
- IT+UT suite still 1356 green (no touches to model/view code).

Code architected by Disco DeDisco <discodedisco@outlook.com>
Git commit message Co-Authored-By:
Claude Sonnet 4.6 <noreply@anthropic.com>

src/static_src/scss/_burger.scss

@@ -22,7 +22,10 @@
     position: fixed;
     bottom: 4.2rem;
     left: 0.5rem;
-    z-index: 318;
+    // Match .gear-btn page-level z (314) — keeps burger BELOW kit_btn +
+    // bud_btn (both z-318) + bud_panel (z-317) so those naturally cover
+    // burger when they overlap. Dialog (z-316) covers burger too.
+    z-index: 314;
     font-size: 1.75rem;
     cursor: pointer;
     color: rgba(var(--secUser), 1);
@@ -60,7 +63,7 @@
     left: 2rem;       // burger left   (0.5) + half-btn (1.5)
     width: 0;
     height: 0;
-    z-index: 317;     // below #id_burger_btn so the burger stays clickable
+    z-index: 313;     // below #id_burger_btn (314) so burger stays clickable
     pointer-events: none;
 
     @media (orientation: landscape) {
@@ -119,3 +122,9 @@
     opacity: 1;
     pointer-events: auto;
 }
+
+// No explicit bud_panel / kit_dialog hide rules needed — burger's z=314
+// sits BELOW kit_btn / bud_btn (318), bud_panel (317), + kit_dialog (316),
+// so all those naturally cover the burger when they overlap. Earlier
+// iterations explicitly faded the burger via opacity; rendered redundant
+// by the z-index drop.