Clarify local scope semantics and add explicit type check in SumLocalStep

[xiazcy]

Summary

Fixes a non-deterministic type-safety hole in SumLocalStep and documents the Scope.local boxing semantics in the
provider semantics specification.

Problem

sum(local) on a single non-numeric scalar (e.g. g.inject("hello").sum(local)) would non-deterministically either
throw ClassCastException or pass the value through as identity, depending on JIT compilation state. This is because
the generic bound E extends Number is erased at runtime, and the arithmetic call that would enforce it
(NumberHelper.add) is only reached when there are 2+ elements. mean(local) was already correct (always calls
div()), while min(local)/max(local) are unaffected (they use Comparable bounds, and NumberHelper.min/max
correctly falls back to compareTo for non-Numbers).

Fix

Added an explicit instanceof Number check in SumLocalStep after untilNonNull() returns the first element, making
the error deterministic.

Documentation

Added a "Local scope and single values" section ([[gremlin-semantics-local-scope-boxing]]) to
gremlin-semantics.asciidoc specifying:

• The wrapping dispatch (LIST/SET/array/MAP iterate directly; anything else wraps to a single-element sequence)
• Scalar numeric identity behavior (sum(local) on 29 → 29)
• Type error contract for incompatible input (regardless of collection size)

Tests

• Gherkin (Sum.feature): sum(local) on a non-numeric list (error) and a single non-numeric scalar (error),
  with GLV translations for .NET, Go, JS, Python
• Unit (SumLocalStepTest): numeric scalar identity + non-numeric error
• Unit (MeanLocalStepTest): numeric scalar identity + non-numeric error
• Unit (MinLocalStepTest): numeric identity, String identity (valid Comparable), String-list min, numeric-list
  min
• Unit (MaxLocalStepTest): numeric identity, String identity, String-list max, numeric-list max
• Unit (IteratorUtilsTest): Number singleton wrapping + generic Object singleton wrapping

---

VOTE +1