cargo test on jarvis-core was failing with `LNK1181 cannot open input
file "libvosk.lib"`. The link-search path is declared in jarvis-app's
build.rs (and jarvis-cli's), but the test binary for jarvis-core has
no such config and the workspace .cargo/config.toml didn't cover it.
Adds a minimal build.rs that emits `cargo:rustc-link-search=native=...`
pointing at lib/windows/amd64 (resolved relative to CARGO_MANIFEST_DIR,
no hardcoded absolute paths so the project still builds for anyone
cloning under a different drive/path).
Verified: cargo test -p jarvis-core --lib commands::tests now passes
3/3 (every_command_toml_parses, lua_command_scripts_exist,
every_command_has_phrases).
