Fix Nor 3/4-arity, timer state ordering, empty And/Or and timespan validation

[DevJasperNL]

Summary

Functional bug fixes found in a full review of the library, plus the regression tests that were missing for each of them.

Bugs fixed

Nor with three or four inputs returned OR (BooleanObservableExtensions.Operators.Or.cs)
The 3- and 4-arity Nor overloads were missing the trailing .Not(). With all inputs false they emitted false instead of true. There were no Nor tests at all, which is how it slipped through. Both overloads now delegate to Or(...).Not() exactly like Nand.

Timer and re-entrancy state corruption (TimedBooleanOperator.cs)
LastSourceValue was assigned after the subclass hook ran. Two concrete failures:

• WhenTrueFor(span, Scheduler.Immediate) threw NullReferenceException on the first true, because the immediate scheduler runs the timer inline while LastSourceValue is still null.
• An observer that feeds back into the source (same-thread re-entrancy passes the lock) had its value clobbered when the outer call resumed. With WhenStableFor, a fed-back false was silently lost and the output stayed true.

The value is now recorded before dispatching and the previous value is handed to OnSourceValue(value, previous) explicitly. Unsubscribe also flips the terminated flag before the timer is disposed.

Empty and null inputs to And / Or
CombineLatest over zero sources never emits and never completes, so Array.Empty<IObservable<bool>>().And() was a silent Never. An empty collection now emits true for And and false for Or and completes. A null observable or a null element throws an argument exception at call time instead of a NullReferenceException at subscribe time.

Publish workflow used the wrong tag (.github/workflows/nuget-publish.yml)
The version came from the newest tag in the repository rather than the release that triggered the run, so a patch release on an older line would have been stamped with the wrong version. It now uses github.event.release.tag_name.

Breaking change

Zero or negative timespans now throw ArgumentOutOfRangeException in every scheduling operator, matching what BlinkWhileTrue already did. Previously TrueForAtLeast, PersistTrueFor, WhenTrueFor, WhenStableFor and PulseTrueFor returned the raw source for timeSpan <= Zero, which bypassed distinctUntilChanged and WhenTrueFor's "first output is always false" contract, and LimitTrueDuration accepted any value and silently forced false right after every true for a negative span. The XML docs and the README (Common options) document the new behaviour.

Tests

• Nand / Nor truth tables for every overload (2-, 3-, 4-arity, params, IEnumerable, collection) over all 16 input combinations
• Re-entrant feedback with WhenStableFor
• WhenTrueFor with Scheduler.Immediate
• Empty collection and null inputs for And / Or
• Zero and negative timespan per scheduling operator

All five new behavioural tests fail against the previous library code and pass with this change. Full suite: 619 tests green on net8.0, net9.0 and net10.0, built with TreatWarningsAsErrors=true.