attribution: in-game ODbL credit via AGigLineHUD

The demo is built from OpenStreetMap data (ODbL 1.0) and needs attribution
wherever the derived work is shown. Add it to the running game, not just the
documents.

- AGigLineHUD (C++): plain AHUD::DrawText in the bottom-left draws
  '(c) OpenStreetMap contributors' and 'Data (c) OpenStreetMap contributors,
  ODbL 1.0 - openstreetmap.org/copyright', with a shadow pass for legibility
  over bright pavement and sky. No UMG widget asset, so a cook cannot drop it
  through a missing widget reference.
- AGigLineGameModeBase: HUDClass = AGigLineHUD, so the credit is on by default.
- AGigLineShotActor: screenshots are now requested with the UI shown, so the
  attribution appears in every capture under Screenshots/ and the obligation
  is provable in review rather than only described.

Docs: THIRD_PARTY_NOTICES records that the string is displayed in-game;
Screenshots/README documents the overlay; TODO marks the item done and
narrows the remaining ODbL question to the shipped-derivative-database
posture.

Screenshots/README.md

@@ -56,6 +56,11 @@ All ☑ approved 2026-11-07.
   FBX → Unreal import.
 - **Lighting is dynamic-only** (`r.AllowStaticLighting=False`, all lights
   Movable), so no lighting build is ever required.
+- **Attribution overlay**: every shot carries the required ODbL credit in the
+  bottom-left (`© OpenStreetMap contributors` / `Data © OpenStreetMap
+  contributors, ODbL 1.0 — openstreetmap.org/copyright`), drawn by
+  `AGigLineHUD`. Screenshots are requested with the UI shown so the obligation
+  is visible in review.
 
 ## What to look for