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.

tests/run.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+# Test runner — discovers every tests/**/*.sx, runs each via `$SX run`, and
+# reports per-test ok/FAIL. Exits 0 only when every test passes.
+#
+# A test passes iff `$SX run <file>` exits 0 (compile failures count as a
+# failure too). Each test program asserts its own invariants — e.g. via
+# process.assert — and exits non-zero on failure.
+#
+# Locate the compiler via SX (overridable); defaults to the sibling sx repo.
+set -u
+
+SX="${SX:-/Users/agra/projects/sx/zig-out/bin/sx}"
+TESTS_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+
+pass=0
+fail=0
+
+# Test filenames follow the no-spaces convention, so word-splitting the
+# sorted find output is safe and keeps the loop in the current shell (so the
+# pass/fail counters survive).
+for t in $(find "$TESTS_DIR" -name '*.sx' -type f | sort); do
+    name=${t#"$TESTS_DIR"/}
+    if "$SX" run "$t" >/dev/null 2>&1; then
+        printf '  %-44s ok\n' "$name"
+        pass=$((pass + 1))
+    else
+        printf '  %-44s FAIL\n' "$name"
+        fail=$((fail + 1))
+    fi
+done
+
+total=$((pass + fail))
+if [ "$total" -eq 0 ]; then
+    echo "no tests found under $TESTS_DIR" >&2
+    exit 1
+fi
+
+echo "------------------------------------------------"
+printf 'tests: %d   pass: %d   fail: %d\n' "$total" "$pass" "$fail"
+
+[ "$fail" -eq 0 ]

tests/smoke.sx

@@ -0,0 +1,9 @@
+// Smoke test: proves the sx toolchain is wired end-to-end from this repo —
+// the standard library import resolves, the program runs, and it exits 0.
+// The runner (tests/run.sh) treats a clean exit as `ok`.
+#import "modules/std.sx";
+
+main :: () -> s32 {
+    print("distribution smoke test: toolchain ok\n");
+    return 0;
+}