feat(duckstation): switch to Provenance-Emu fork, update to latest master

- Update .gitmodules URL from stenzek/duckstation to Provenance-Emu/duckstation
- Pin submodule to 125c3ec (2026-03-20: "Qt: Respond to QScreen refresh rate changes")
- Add branch = master tracking in .gitmodules

Part of #3447

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: github-actions[bot]

.changelog/3447.md

@@ -0,0 +1,2 @@
+### Changed
+- **DuckStation submodule** — Switched submodule URL from upstream `stenzek/duckstation` to the Provenance-Emu fork and updated to latest master (2026-03-20).

.gitmodules

@@ -30,7 +30,8 @@
 	url = https://JoeMatt@github.com/Provenance-Emu/dolphin-ios-jitless.git
 [submodule "Cores/DuckStation/duckstation"]
 	path = Cores/DuckStation/duckstation
-	url = https://github.com/stenzek/duckstation.git
+	url = https://github.com/Provenance-Emu/duckstation.git
+	branch = master
 [submodule "Cores/Desmume2015/desmume2015"]
 	path = Cores/Desmume2015/desmume2015
 	url = https://github.com/Provenance-Emu/desmume2015-1.git