fix: Guid non-unique indexes + portable Nullable<T> index record loss

[OmidID]

Two independent index bugs, plus regression tests.

Embedded — non-unique secondary index over a Guid-keyed table threw (NullReferenceException at insert, InvalidOperationException at comparer build). A non-unique index key is Slots(field, primaryKey); with a Guid PK that composite contains a Guid slot, which had no comparer:

• TypeEngine only built comparers for all-primitive or bare-Guid types, so the composite got a null KeyComparer -> NRE. Add DataTypeUtils.IsAllComparable (primitive-or-Guid, recursive) and widen the gate (IndexerPersist stays primitive-only — Guid has none).
• ComparerHelper declared the shared cmp temp only when a char/byte[]/ decimal/string member was present, so a composite whose members are Guid/ int/DateTime referenced an undeclared variable and the comparer expression would not compile. Always declare cmp.

Remote/portable — a non-unique index over a Nullable field SILENTLY dropped main-table rows (100 inserts -> ~2 survive, no exception). The server opens tables portable, so a Nullable field arrives as a nested single-slot Slots; the index built a malformed key Slots(Slots, pk) -> null KeyComparer -> NRE swallowed server-side -> the write batch was dropped. General remote Query on the field additionally threw "The type (T) is not primitive".

• SlotAccessor: treat a single-slot Slots (the portable Nullable
  representation) as its inner T in NormalizeStorageType / NormalizeStorageValue
  (null wrapper -> default(T)) and BuildValueNormalizer; add
  TryGetPortableNullableType.
• TableIndexManager.GetMemberType: reconstruct Nullable from the portable slot so the remote field codec stays symmetric with the client.

Tests: GuidPkNonUniqueReproTests, RemoteInsertReproTests, RemoteNullableQueryTests. Full suite 360 passing; live-server stress run reports zero errors.