chore: project scaffold and build wiring

Initial Zig 0.16 build script wiring the public `zbench` module, the
`zbench_build` helper module for downstream `build.zig` files, an
internal `test` step, and an `example` step that compiles and runs the
bundled benchmark suite.

.gitignore

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+.zig-cache/
+zig-out/

build.zig

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+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    // The public `zbench` module: what dependents @import.
+    const zbench_module = b.addModule("zbench", .{
+        .root_source_file = b.path("src/root.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+
+    // The `zbench_build` module: build-time helpers (add_bench_step).
+    // Dependents @import("zbench_build") from their build.zig.
+    _ = b.addModule("zbench_build", .{
+        .root_source_file = b.path("src/build_helper.zig"),
+    });
+
+    // Internal unit tests: `zig build test`.
+    const tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/root.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
+    });
+    const run_tests = b.addRunArtifact(tests);
+    const test_step = b.step("test", "Run zbench unit tests");
+    test_step.dependOn(&run_tests.step);
+
+    // Convenience: build the bundled example.
+    const example_exe = b.addExecutable(.{
+        .name = "zbench-example",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("examples/bench/main.zig"),
+            .target = target,
+            .optimize = .ReleaseFast,
+        }),
+    });
+    example_exe.root_module.addImport("zbench", zbench_module);
+    b.installArtifact(example_exe);
+
+    const run_example = b.addRunArtifact(example_exe);
+    if (b.args) |args| run_example.addArgs(args);
+    const example_step = b.step("example", "Run the bundled example benchmarks");
+    example_step.dependOn(&run_example.step);
+}

build.zig.zon

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+.{
+    .name = .zbench,
+    .version = "0.1.0",
+    .fingerprint = 0xbb819b3c1603b711,
+    .minimum_zig_version = "0.16.0",
+    .paths = .{
+        "build.zig",
+        "build.zig.zon",
+        "src",
+        "examples",
+    },
+    .dependencies = .{},
+}