The DMG, p290, says
If a character’s Constitution score drops, then he loses 1 hit point per Hit Die for every point by which his Constitution modifier drops. For example, at 7th level, Tordek is hit by poison that causes his Constitution to drop from 16 to 13. His Constitution modifier falls from +3 to +1, so he loses 14 hit points (2 per level). A minute later, the poison deals another 8 points of Constitution damage, dropping his score to 5 and his modifier to –3. He loses another 28 hit points—for a total of 42 hit points lost because of an overall 6-point drop in his Constitution modifier.
A hit point score can’t be reduced by Constitution damage or drain to less than 1 hit point per Hit Die. At 7th level, Mialee has 22 hit points when fully healed. Even if her Constitution score drops to 5 or lower, she will still have at least 7 hit points (less any damage she may take).
I cannot find any other use of the phrase "hit point score"; that paragraph is all we get. However: the parenthetical says "she may take", not "she may have taken". Thus, I think that the intent is that con damage/drain can't kill you from HP loss (getting your con to 0 will still kill you, of course).
Thus, I think the general rules are:
- con damage can't bring your current HP below your number of HD
- con damage can't bring your max HP below your number of HD
- con damage may result in your current HP equaling your max HP, preventing healing HP (eg., from cure spells)
- healing con damage can't bring your current/max HP above your normal max
So, Steve's odyssey, with his 5 HD:
|arsenic burrito; -9 con. Max/Current HP hit the "1/HD" floor.
|lesser restoration, +3 con. Con mod increases by 2, so Steve regains 2*<hd> = 10 HP.
|new event lesser restoration, +4 con. Con mod increases by 2 again, so Steve regains another 2*<hd> = 10 hp, but the increase is capped at his "full normal hit points".
|Fully recovered; back to baseline.
|Sassone leaves, 10 hp damage.
|Sassone leaves, 6 con damage.
|lesser restoration, +2 con.
Yes, that does mean that Steve's current HP are higher after bouncing through some con damage than if he hadn't had that damage; healing the con rather than healing the hp damage is a more efficient option (at least in terms of count of actions). I think that's fine, since it's (IME) a pretty rare edge case.
The quotes below, I believe, support that table: the rules around tracking HP. Specifically, p134's "damage" section ("Damage reduces a target’s current hit points") - you don't track "I have 18 HP and 7 damage", you track "my full maximum HP is 18, but I currently have 11 HP" (note that nonlethal damage works differently: you track "I currently have 7 nonlethal damage", so you can compare your nonlethal damage against your current HP total to see if you're unconscious).
The Rules Compendium, on P72, does either clarify or contradict the above. I'm not sure whether the compendium takes precedence over the core books (which is somewhat to say whether it's considered errata to the core books, which would give it precedence, or not). But, it does simplify things awkwardly:
If a creature’s Constitution score increases, that creature gains 1 hit point per Hit Die for every point by which its Constitution modifier goes up. Such hit points aren’t the same as temporary hit points. They can be healed with magic and even naturally, if the effect that caused them to appear remains long enough.
If a creature’s Constitution score drops, that creature loses 1 hit point per Hit Die for every point by which its Constitution modifier drops. Loss of Constitution can’t reduce a creature’s full normal hit points to less than 1 hit point per Hit Die. It can reduce a wounded creature’s current hit points to 0 or fewer. A creature reduced to 0 Constitution dies regardless of its current hit points.
Using the compendium, Steve goes unconscious (with -7 HP) after the arsenic burrito and the sassone leaf con damage. ... assuming "wounded" means "taken con damage" and not just "taken HP damage", in which case the pre-compendium analysis remains intact until the sassone leaf con damage knocks Steve unconscious. FWIW, this GM has never had a problem with the former interpretation - in fact: this GM hadn't remembered the "minimum 1 HP/HD after con damage" rule, and has never had problems from con damage killing characters from HP loss.
Other uses of "hit points" that are relevant, all from the PHB; I think that, together, they corroborate my interpretation.
PHB p9, under the description of con says
Each roll of a Hit Die (though a penalty can never drop a result below 1—that is, a character always gains at least 1 hit point each time he or she advances in level).
If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly.
The "changing ability scores" section says that
When an ability score changes, all attributes associated with that
score change accordingly.
(then goes on to talk about changes to int, unhelpfully here).
p58, "level advancement" says
- Hit Points: Roll a Hit Die, add your character’s Constitution
modifier, and add the total roll to his or her hit points. Even if the character has a Constitution penalty and the roll was so low as to yield a result of 0 or fewer hit points, always add at least 1 hip point upon gaining a new level.
p135 expands slightly on how damage works:
If you score a hit, roll damage and deduct it from the target’s current hit points.
p139, talking about the attack action, concurrs:
Damage is deducted from the target’s current hit points.
p146, in the temporary hit points section
Increases in Constitution Score and Current Hit Points: An increase in a character’s Constitution score, even a temporary one, can give her more hit points (an effective hit point increase), but these are not temporary hit points. They can be restored, such as with cure light wounds, and they are not lost first as temporary hit points are. For example, Krusk (now a 3rd-level barbarian) gains +4 to his Constitution score and +6 hit points when he rages, raising his hit points from 31 to 37. If Krusk takes damage dropping him to 32 hit points, Jozan can cure those lost points and get him back to 37. If Krusk is so wounded at the end of his rage that he only has 5 hit points left, then when he loses his 6 extra hit points, he drops to –1 hit points and is dying.
p146, in the nonlethal damage section, says
When you take nonlethal damage, keep a running total of how much you’ve accumulated. Do not deduct the nonlethal damage number from your current hit points. It is not “real” damage. Instead, when your nonlethal damage equals your current hit points, you’re staggered, and when it exceeds your current hit points, you fall unconscious. It doesn’t matter whether the nonlethal damage equals or exceeds your current hit points because the nonlethal damage has gone up or because your current hit points have gone down.
current hit points: A character’s hit points at a given moment in the game. Current hit points go down when the character takes damage and go back up upon recovery.
full normal hit points: An individual character’s maximum hit
points when undamaged.
hit points (hp): A measure of a character’s health or an object’s integrity. Damage decreases current hit points, and lost hit points return with healing or natural recovery. A character’s hit point total increases permanently with additional experience and/or permanent increases in Constitution, or temporarily through the use of various special abilities, spells, magic items, or magical effects (see temporary hit points and effective hit point increase).