Cap CompactArcStore state count by StateId and validate header counts before narrowing.

[copybara-service]

Cap CompactArcStore state count by StateId and validate header counts before narrowing.

In CompactArcStore<Element, Unsigned> (lib/compact-fst.h):

1. CompactArcCompactor::NumStates() returns StateId (int32_t for StdArc,
   max 2^31 - 1), whereas kMaxStates is 1ULL << 52 (CompactArcStore is
   not templated on Arc, but its constructors and Read() are templated on
   ArcCompactor).
2. In CompactArcStore::Read, the 64-bit signed hdr.NumStates() and
   hdr.NumArcs() values were assigned to unsigned size_t (data->nstates_
   and data->narcs_) before checking bounds, making data->nstates_ < 0 and
   data->narcs_ < 0 dead unsigned comparisons.

Fix by:

• Making kMaxStates and kMaxArcs public on CompactArcStore.
• Capping the maximum allowed state count in CompactArcStore's constructors
  and Read() to std::min<uint64_t>(kMaxStates, std::numeric_limits<StateId>::max()).
• Validating hdr.NumStates(), hdr.Start(), and hdr.NumArcs() as int64_t
  in CompactArcStore::Read before assigning them to data->nstates_,
  data->start_, and data->narcs_.
• Expanding CompactLimitsTest in fst_limits_test.cc to test
  std::numeric_limits<StdArc::StateId>::max() + 1 and negative counts.