rust-embed-rebuild-invalidation

Force rust-embed to re-embed assets from $GRAFEMA_UI_DIST (or any env-driven path) when cargo's incremental compilation skips the macro re-expansion. Use when: (1) you rebuilt the GUI bundle but the rust-server binary still serves the OLD bundle, (2) cargo build --release finishes in seconds (incremental hit) and strings target/release/binary | grep <new-hash> finds nothing, (3) any rust-embed pipeline where assets live outside the cargo source tree and are picked up via env var or symlink. Cargo only re-runs the proc macro when the rust file declaring #[derive(RustEmbed)] changes — env var changes alone don't trigger it.