Add Clair Obscur: Expedition 33 via new METHOD=external

Expedition 33 is UE5 with no DRM, but a mature MIT-licensed fix already exists:
ClairObscurFix by Lyall. Rather than reimplement it, install.sh gains a third
method that vendors a third-party fix unmodified and layers our own tuned
config on top, with the upstream license shipped in the release zip as MIT
requires. Upstream is archived, so vendoring also keeps it available.

Our only change vs upstream defaults is [Center HUD] Enabled = true, since at
5120px wide the edge-anchored HUD elements end up absurdly far apart.

Install/uninstall round-trip verified against a scratch game dir, including
that our tuned ini overrides the vendored default.

Credit to Lyall is recorded in THIRD_PARTY.md, the game README, and the repo
README's method table.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

vendor/ClairObscurFix/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025 Lyall
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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+SOFTWARE.