Add Tower Defence environment

[Kbediako]

tower-defence-native-live-policy-manual-takeover-wslg-20260723T013622Z.mp4

Adds Tower Defence as a single-agent native C environment.

• 5,686 observations with 3,572 masked actions across 510 placement sites.
• Includes original assets and a playable demo with live policy inference or hold-Shift manual control.
• Bundles a 120M-step MinGRU checkpoint averaging 461/500 across six public evaluation seeds.
• Adds environment tests, render tests, and training configuration.

Validated with strict C compilation, sanitizers, and Valgrind.

Depends on #616 for lossless transport of large discrete action IDs.