Tvheadend

Default installation from deb package user hts is created and TVHeadend runs under these. TVHeadend can be run as any user or group you like, just don't use root. If it is being started as a service look to configuration file /etc/default/tvheadend. Edit

TVH_USER="hts"

TVH_GROUP="video"

and change to user and group you prefer. Restart tvheadend service. I just run TVH under my own user account and group video. .hts is in my home dir. Not a public system so handier for me.

Should write to any share once it's mounted as long as permissions are given. Can also use NFS share if available on your NAS.

"which I think means it's running as user hts"

Correct.

"Is the hts user created by the installation in order to run Tvheadend as a service?"

Yes, (presumably!)

"I noticed that the file/directory permissions are 'in Linux'. Will Tvheadend write to an SMB share if it's mounted?"

I'm not familiar with SMB. A quick search says the answer is yes providing it is not mounted read-only on the SMB side and the linux permissions are set properly on the mounted directory.

You can check what permissions are seen on the linux system with 'ls -l'. If you need to change ownership then you'll need to use the chown command and you'll need to be root (i.e. sudo) to do it.

"Final general question: I thought I had read that GIDs below 1000 were reserved for system use and had interpreted that to mean by the builders of Linux. Am I to reinterpret that as meaning anything that doesn't require an interactive logon?"

No, it means a 'system' user as opposed to a 'living breathing' user. Google will give you a better and more complete answer.

A few years ago I made a guide for myself (and others) about how I used to run Tvheadend on Debian in a VM running on hyper-v, I like you mostly use Windows.
The guide is here: https://tvheadend.org/boards/4/topics/28598?r=35470#message-35470
And in that guide there is a section on using automount to a share on a windows pc from the linux machine.
I'm not sure any of that is of much use to you but read it and things might work, best of luck

Here's the snip from that guide:
As I wrote above I use a Windows machine to store all larger recordings, its a Win10 called mypc and have ip 192.168.1.2 with a share called recordedtv, ther is a local user on it called kodi with the passwork kodi and this user have full permissions on the share and folder on the machine.
I dont want this to be permanently attachaed - so the solution is autofs.

To add recordings to my Windows machine here is how:
sudo apt-get install cifs-utils autofs
sudo mkdir /mnt/mypc
sudo chmod 7777 /mnt/mypc

Add this at the bottom of the file and comment the line +auto.master and paste the info
sudo nano /etc/auto.master
#+auto.master
/mnt/mypc /etc/auto.mypc --timeout=60 --ghost
To save: ctrl+o (enter)
To exit: ctrl+x

sudo id hts
Replace uid and gid with what the user hts have on you system and then paste the line
sudo nano /etc/auto.mypc
recordedtv -fstype=cifs,rw,username=kodi,password=kodi,uid=107,gid=111 ://192.168.1.2/recordedtv
To save: ctrl+o (enter)
To exit: ctrl+x

Restart it:
sudo /etc/init.d/autofs restart

In TVH webgui enter this as recordingpath to store recordings on you Win PC:
/mnt/mypc/recordedtv