Freeciv

That's unlikely to be the only place where such a zombie (unit dies when it reach 0 HP) causes problems. The fix should be in ruleset sanity checking.

secfile_lookup_int_default_min_max() would be your friend.

For rulesets that play excessively with better firepower and hitpoints as game progresses, maximum hp of 100 is rather low upper limit, especially in S3_2 where we should not break any existing rulesets (d3f). I'd say 10000 would be safe value even as such absolute maximum.

You should also make it impossible to set hitpoints to zero (or other illegal value) from the ruledit. Otherwise one may accidentally changes hitpoints to zero, saves the ruleset, and the ruleset load time check makes it impossible to load ruleset back in for fixing the situation.

We don't want to knowingly introduce the bug in the git history, and only separately fix it. Please combine ruledit change with the rest of the patch (single patch for each branch)

The patch does not apply to main branch. The same patch does apply for S3_3, so I guess you just need a separate patch for main.