createWindowsBlock

    pub fn createWindowsBlock(
        map: *const Map,
        gpa: Allocator,
        options: CreateWindowsBlockOptions,
    ) error{ OutOfMemory, InvalidWtf8 }!WindowsBlock {
        // count bytes needed
        const max_chars_needed = max_chars_needed: {
            var max_chars_needed: usize = "\x00".len;
            if (options.zig_progress_handle) |handle| if (handle != std.os.windows.INVALID_HANDLE_VALUE) {
                max_chars_needed += std.fmt.count("ZIG_PROGRESS={d}\x00", .{@intFromPtr(handle)});
            };
            for (map.keys(), map.values()) |key, value| {
                if (options.zig_progress_handle != null and eqlKeys(key, "ZIG_PROGRESS")) continue;
                max_chars_needed += key.len + "=".len + value.len + "\x00".len;
            }
            break :max_chars_needed @max("\x00\x00".len, max_chars_needed);
        };
        const block = try gpa.alloc(u16, max_chars_needed);
        errdefer gpa.free(block);

        var i: usize = 0;
        if (options.zig_progress_handle) |handle| if (handle != std.os.windows.INVALID_HANDLE_VALUE) {
            @memcpy(
                block[i..][0.."ZIG_PROGRESS=".len],
                &[_]u16{ 'Z', 'I', 'G', '_', 'P', 'R', 'O', 'G', 'R', 'E', 'S', 'S', '=' },
            );
            i += "ZIG_PROGRESS=".len;
            var value_buf: [std.fmt.count("{d}", .{std.math.maxInt(usize)})]u8 = undefined;
            const value = std.mem.print(&value_buf, "{d}", .{@intFromPtr(handle)}) catch unreachable;
            for (block[i..][0..value.len], value) |*r, v| r.* = v;
            i += value.len;
            block[i] = 0;
            i += 1;
        };
        for (map.keys(), map.values()) |key, value| {
            if (options.zig_progress_handle != null and eqlKeys(key, "ZIG_PROGRESS")) continue;
            i += try unicode.wtf8ToWtf16Le(block[i..], key);
            block[i] = '=';
            i += 1;
            i += try unicode.wtf8ToWtf16Le(block[i..], value);
            block[i] = 0;
            i += 1;
        }
        // An empty environment is a special case that requires a redundant
        // NUL terminator. CreateProcess will read the second code unit even
        // though theoretically the first should be enough to recognize that the
        // environment is empty (see https://nullprogram.com/blog/2023/08/23/)
        for (0..2) |_| {
            block[i] = 0;
            i += 1;
            if (i >= 2) break;
        } else unreachable;
        const reallocated = try gpa.realloc(block, i);
        return .{ .slice = reallocated[0 .. i - 1 :0] };
    }