chore: add root Makefile with namespaced targets

Namespaced go-* targets (build, run, test, test-verbose, check, cover,
fmt, vet, tidy, install, lint, clean) plus umbrella targets that delegate
to them. help is the default. py-* targets will slot in during the Python
port. Updates .gitignore with coverage artifacts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile

@@ -0,0 +1,91 @@
+GO_DIR  := go
+BINARY  := latprobe
+ARGS    ?=
+
+.DEFAULT_GOAL := help
+
+# ── help ──────────────────────────────────────────────────────────────────────
+
+.PHONY: help
+help: ## Show this help
+	@grep -E '^[a-zA-Z_-]+:.*##' $(MAKEFILE_LIST) \
+		| awk 'BEGIN {FS = ":.*##"}; {printf "  \033[36m%-20s\033[0m %s\n", $$1, $$2}' \
+		| sort
+
+# ── umbrella targets (delegate to Go now; py-* will be added during Python port) ──
+
+.PHONY: all
+all: check build ## Run checks then build
+
+.PHONY: build
+build: go-build ## Build binary (delegates to go-build)
+
+.PHONY: test
+test: go-test ## Run tests (delegates to go-test; py-test added later)
+
+.PHONY: check
+check: go-check ## Run fmt + vet + test gate (delegates to go-check)
+
+.PHONY: fmt
+fmt: go-fmt ## Format source code (delegates to go-fmt)
+
+.PHONY: vet
+vet: go-vet ## Run go vet (delegates to go-vet)
+
+.PHONY: clean
+clean: go-clean ## Remove build and coverage artifacts
+
+# ── Go targets ────────────────────────────────────────────────────────────────
+
+.PHONY: go-build
+go-build: ## go: build binary → go/latprobe
+	go -C $(GO_DIR) build -o $(BINARY) .
+
+.PHONY: go-run
+go-run: ## go: build and run (pass flags via ARGS="…")
+	go -C $(GO_DIR) run . $(ARGS)
+
+.PHONY: go-test
+go-test: ## go: run all tests
+	go -C $(GO_DIR) test ./...
+
+.PHONY: go-test-verbose
+go-test-verbose: ## go: run all tests with per-case output
+	go -C $(GO_DIR) test -v ./...
+
+.PHONY: go-check
+go-check: go-fmt go-vet go-test ## go: fmt + vet + test (pre-commit gate)
+
+.PHONY: go-cover
+go-cover: ## go: run tests with coverage → go/coverage.html
+	go -C $(GO_DIR) test -coverprofile=coverage.out ./...
+	go -C $(GO_DIR) tool cover -html=coverage.out -o coverage.html
+	@echo "Coverage report: $(GO_DIR)/coverage.html"
+
+.PHONY: go-fmt
+go-fmt: ## go: format all Go source files with gofmt
+	gofmt -w $(GO_DIR)
+
+.PHONY: go-vet
+go-vet: ## go: run go vet on all packages
+	go -C $(GO_DIR) vet ./...
+
+.PHONY: go-tidy
+go-tidy: ## go: run go mod tidy
+	go -C $(GO_DIR) mod tidy
+
+.PHONY: go-install
+go-install: ## go: install binary to $GOBIN / $GOPATH/bin
+	go -C $(GO_DIR) install .
+
+.PHONY: go-lint
+go-lint: ## go: run golangci-lint (must be installed)
+	@command -v golangci-lint >/dev/null 2>&1 || { \
+		echo "golangci-lint not installed — see https://golangci-lint.run/usage/install/"; \
+		exit 1; \
+	}
+	cd $(GO_DIR) && golangci-lint run
+
+.PHONY: go-clean
+go-clean: ## go: remove binary and coverage artifacts
+	rm -f $(GO_DIR)/$(BINARY) $(GO_DIR)/coverage.out $(GO_DIR)/coverage.html