Fix reagent temperature mismatch crash (#23063)

* Please describe the intent of your changes in a clear fashion.
This PR addresses a crash (`SERVER-PROD-2R6`) caused by a temperature
mismatch when adding new reagents, particularly in low-volume metabolism
holders.

The root cause was identified as a combination of:
1. A coarse rounding guard in `set_thermal_energy` that discarded
small-but-valid thermal energy changes.
2. A temperature assertion in `add_reagent` that was too sensitive to
floating-point inaccuracies.
3. The `nicotine` reagent lacking an explicit `fallback_specific_heat`.

Changes implemented:
- **`code/modules/reagents/Chemistry-Temperature.dm`**: Modified
`set_thermal_energy` to remove the `round(delta, 1)` guard. Now,
`if(!delta)` is used, allowing small, non-zero energy changes to be
processed.
- **`code/modules/reagents/Chemistry-Holder.dm`**: Updated the
temperature assertion in `add_reagent` from `round(temperature, 1) !=
round(get_temperature(), 1)` to `abs(temperature - get_temperature()) >
0.1`. This provides a more robust, tolerance-based comparison.
-
**`code/modules/reagents/Chemistry-Reagents/Chemistry-Reagents-Medicine.dm`**:
Added `fallback_specific_heat = 1.67` to
`/singleton/reagent/mental/nicotine` to ensure it has a proper specific
heat value.

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### Asset Licenses
The following assets that **have not** been created by myself are
included in this PR:

| Path | Original Author | License |
| --- | --- | --- |
| icons/example.dmi | ExamplePerson (Example Station) | CC0 |


Fixes
[SERVER-PROD-2R6](https://aurorastation.sentry.io/issues/7661064998/?seerDrawer=true)

---------

Co-authored-by: VMSolidus <evilexecutive@gmail.com>

code/modules/reagents/Chemistry-Holder.dm

@@ -135,7 +135,7 @@
 			newreagent.set_thermal_energy(new_thermal_energy, src, safety = TRUE)
 		else
 			newreagent.set_temperature(temperature, src, safety = TRUE)
-		if(!new_thermal_energy && round(temperature, 1) != round(get_temperature(), 1))
+		if(!new_thermal_energy && abs(temperature - get_temperature()) > 0.1)
 			crash_with("Temperature [temperature] did not match [get_temperature()] after adding NEW reagent [rtype]!")
 	else	// Existing reagent
 		var/old_energy = (newreagent.get_thermal_energy(src)/reagent_volumes[rtype]) * amount

code/modules/reagents/Chemistry-Reagents/Chemistry-Reagents-Medicine.dm

@@ -1280,6 +1280,7 @@
 	goodmessage = list("You feel good.","You feel relaxed.","You feel alert and focused.")
 	value = 2
 	alchohol_affected = FALSE
+	fallback_specific_heat = 1.67 // Approximate specific heat of liquid nicotine (J/g·K)
 
 /singleton/reagent/mental/nicotine/overdose(var/mob/living/carbon/M, var/alien, var/removed, var/scale, var/datum/reagents/holder)
 	..()

code/modules/reagents/Chemistry-Temperature.dm

@@ -13,7 +13,7 @@
 
 /singleton/reagent/proc/set_thermal_energy(amount, var/datum/reagents/holder, safety = FALSE)
 	var/delta = amount / get_thermal_fraction(holder) - holder.thermal_energy
-	if(!round(delta, 1))
+	if(!delta)
 		return
 	holder.add_thermal_energy(delta, safety) // this handles on_heat_change for us