Fix LongIndexed.compareTo overflow for large index deltas #936

Context

LongIndexed<T> (:utils, utils/src/commonMain/kotlin/it/unibo/tuprolog/utils/LongIndexed.kt) is a Comparable value ordered by its Long index. The interface declares a correct default:

override fun compareTo(other: LongIndexed<T>): Int = index.compareTo(other.index)

but the only implementation, LongIndexedImpl (utils/src/commonMain/kotlin/it/unibo/tuprolog/utils/impl/LongIndexedImpl.kt), overrides it with a subtraction-based comparison:

override fun compareTo(other: LongIndexed<T>): Int = (index - other.index).toInt()

Problem

(a - b).toInt() truncates the Long difference to its low 32 bits, so the sign of the result is wrong whenever |a - b| >= 2^31 (and the subtraction itself can overflow for operands near Long.MIN_VALUE/Long.MAX_VALUE). Example: index = 2^32, other.index = 0 gives (2^32).toInt() == 0, i.e. the two values compare as equal; index = 2^31 vs 0 compares as less than.

Expected: LongIndexed.of(a, x).compareTo(LongIndexed.of(b, y)) has the same sign as a.compareTo(b) for all Long values.

Affected code

• :utils — LongIndexedImpl.compareTo (used by Sequence.longIndexed() in IterUtils.kt).
• :theory — IndexedClause (theory/src/commonMain/kotlin/it/unibo/tuprolog/collections/rete/custom/clause/IndexedClause.kt) extends LongIndexed<Clause> and wraps LongIndexed.of(...), so the RETE-based clause ordering (e.g. mergeSequences(..., SituatedIndexedClause::compareTo) in rete/custom/Utils.kt) inherits the bug for theories whose clause indices diverge by more than Int.MAX_VALUE.

Possible approach

Delete the compareTo override from LongIndexedImpl so the correct interface default (index.compareTo(other.index)) applies, and add a regression test in :utils comparing indices far apart (e.g. 0 vs 1L shl 32, Long.MIN_VALUE vs Long.MAX_VALUE).

Original description

LongIndexed.compareTo can overflow and misorder values when index deltas are large. Spotted during the documentation pass; not fixed in commit 5dfb787.