Move Odin renderer into gui/ with per-project Makefiles

Each sub-project (gui, api, infra) now owns its own Makefile, and the
root Makefile is a lean delegator for the base commands (run, build,
test, clean, fmt). Odin source, deps, and web assets live under gui/
(gui/src, gui/lib, gui/www), with scripts kept under gui/scripts for
build tooling only.

gui/Makefile

@@ -0,0 +1,46 @@
+# gui/Makefile — the Odin + raylib renderer (the interactive 3D universe map).
+#
+# This is the Odin sub-project. It is normally invoked from the repo root via
+# `make run`, `make build`, etc. (which delegate here through the root
+# Makefile), but it can also be driven directly with `make -C gui <target>`.
+#
+# `lib/` (third-party Odin deps) lives under `gui/lib`; the `bin/` / `build/`
+# output dirs live at the repo root and are referenced through `$(ROOT)`.
+
+ODIN         ?= odin
+ROOT         := ..
+BIN          := $(ROOT)/bin
+BINARY       := $(BIN)/desi_explorer
+ODIN_FLAGS   := -collection:lib=lib/local
+WASM_DEFINE  := RAYLIB_WASM_LIB=env.o
+
+.PHONY: help run build build-debug build-web test clean fmt
+
+help: ## List available targets
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | \
+		awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[36m%-18s\033[0m %s\n", $$1, $$2}'
+
+run: ## Run the native app
+	@mkdir -p lib/local
+	$(ODIN) run src $(ODIN_FLAGS)
+
+build: ## Release build -> bin/desi_explorer
+	@mkdir -p lib/local $(BIN)
+	$(ODIN) build src $(ODIN_FLAGS) -o:speed -out:$(BINARY)
+
+build-debug: ## Debug build -> bin/desi_explorer
+	@mkdir -p lib/local $(BIN)
+	$(ODIN) build src $(ODIN_FLAGS) -o:none -debug -out:$(BINARY)
+
+build-web: ## WebAssembly build -> build/web (needs emscripten)
+	@scripts/build_web.sh
+
+test: ## Run Odin unit tests
+	@mkdir -p lib/local
+	$(ODIN) test src $(ODIN_FLAGS)
+
+clean: ## Remove build artifacts
+	rm -rf $(BIN) build
+
+fmt: ## Format Odin sources with odinfmt
+	odinfmt src