attribution: in-game ODbL credit via AGigLineHUD

Completes "Geographic attribution display" for 0.1.0. The demo is built from
OpenStreetMap data (ODbL 1.0) and needs attribution wherever the derived work
is shown, so the credit now lives in 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 so it stays
  legible over bright pavement and sky. No UMG widget asset is involved, so a
  cook cannot drop the attribution through a missing widget reference.
- AGigLineGameModeBase: HUDClass = AGigLineHUD, so the credit is on by default
  in both the DemoMap and the ReviewMap.
- AGigLineShotActor: screenshots are requested with the UI shown, so the credit
  appears in every capture under Screenshots/ - the obligation is provable from
  the images rather than only described.

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

Squash of branch gameplay-attribution-0.1.0 (PR #4).

THIRD_PARTY_NOTICES.md

@@ -25,6 +25,8 @@ Status values: **verified** (licence confirmed and permits our use), **pending**
 
 Attribution string for any OSM-derived output: "© OpenStreetMap contributors", <https://www.openstreetmap.org/copyright>. The data is available under the [Open Database License 1.0](https://opendatacommons.org/licenses/odbl/1-0/).
 
+This attribution is **displayed in the running game**, not only in this document: `AGigLineHUD` (`Game/GigLine/Source/GigLine/GigLineHUD.cpp`) draws the credit and the licence URL in the bottom-left of every frame, so the obligation is met by the shipped build. It is visible in every capture under `Screenshots/` because the screenshot actor requests screenshots with the UI shown.
+
 ## Reference photography (local only, not committed, not shipped)
 
 Reference photographs are **not** part of this repository. They live in the gitignored `Art/reference/` tree on the development machine, are used only for internal, private development, and are never built into a build or shipped. `.gitignore` enforces this; nothing under `Art/reference/` may be added, staged, or committed.