Add BakkesMod in-game UI

[colonelpanic8]

Summary

• adds the BakkesMod in-game UI/window system for subtr-actor replay annotations and stats surfaces
• reuses the normal replay-processing analysis graph through the Rust DLL and exposes stats/event data in game
• scopes the in-game launcher to plugin-owned surfaces; Rocket League handles camera/playback, and replay annotations follow the current replay rather than a multi-replay queue
• adds Nix-based Linux DLL build support and source/export verifiers for the plugin

Validation

• python3 bakkesmod/verify-plugin-source.py
• git diff --check origin/master..HEAD
• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• env XDG_CACHE_HOME=/tmp/subtr-actor-nix-cache RUST_MIN_STACK=67108864 CARGO_BUILD_JOBS=1 nix build .#bakkesmod-plugin
• python3 bakkesmod/verify-rust-dll-exports.py result/subtr_actor_bakkesmod.dll result/bakkesmod-install/data/subtr-actor/subtr_actor_bakkesmod.dll
• size scan: no files over 5 MB outside ignored build/dependency dirs