P1.1: repo skeleton + sx build/test gate

Stand up the foundation every later step depends on:

- Source layout: src/, src/infra/, tests/, examples/ (.gitkeep markers).
- Makefile: `build` compiles the smoke program via $SX, `test` runs the
  runner over tests/**/*.sx, `publish-example` placeholder (real in P3.4).
  Compiler located via `SX ?= /Users/agra/projects/sx/zig-out/bin/sx`.
- tests/run.sh: POSIX-sh runner; discovers tests/**/*.sx, runs each via
  `$SX run`, prints ok/FAIL, exits 0 only when all pass (errors on zero
  tests so the gate is never silently empty).
- tests/smoke.sx: passing smoke test importing modules/std.sx — proves
  toolchain wiring end-to-end (std resolves via the binary's own location).
- .gitignore: ignore build/ artifacts.

Makefile

@@ -0,0 +1,29 @@
+# distribution — build/test gate.
+#
+# The sx compiler lives in a separate repo; locate it via SX (overridable):
+#   make build SX=/path/to/sx
+SX ?= /Users/agra/projects/sx/zig-out/bin/sx
+
+BUILD_DIR := build
+
+# Programs compiled by `make build`. Currently just the smoke program;
+# product entry points under src/ get added here as they land (P1.2+).
+SMOKE := tests/smoke.sx
+
+.PHONY: build test publish-example clean
+
+# Compile the smoke program (and, later, product sources) without running.
+build:
+	@mkdir -p $(BUILD_DIR)
+	$(SX) build -o $(BUILD_DIR)/smoke $(SMOKE)
+
+# Run the test runner over every tests/**/*.sx. Exits non-zero on any failure.
+test:
+	@SX="$(SX)" ./tests/run.sh
+
+# Placeholder for the end-to-end publish flow — becomes real in P3.4.
+publish-example:
+	@echo "publish-example: not implemented yet (becomes real in P3.4)"
+
+clean:
+	@rm -rf $(BUILD_DIR)