feat(core)!: support nested map expressions and import nested structs

[nielspardon]

Proto Expression.Nested has three arms — struct, list and map — but only list round-tripped. Expression.NestedMap did not exist at all, so the map arm was unreachable, and ProtoExpressionConverter.from(Nested) handled only case LIST: a plan carrying a nested struct failed on import with UnsupportedOperationException: Unimplemented nested type: STRUCT, even though the POJO to proto direction emitted one. (VirtualTableScan rows were unaffected — they use the separate from(Nested.Struct) overload.)

This adds the NestedMap POJO with the usual visitor and converter wiring, and imports all three nested kinds on the way back from proto.

Two details worth calling out:

• Key order is part of the value. Proto Nested.Map is a repeated key_values, so NestedMap keeps insertion order — the Immutables-generated map is LinkedHashMap-backed — and both converters preserve it. Without that, a round trip could reorder the pairs.
• @Value.Check throws rather than asserts. NestedList, written earlier, uses assert for the same invariants, which does not run outside a -ea JVM. NestedMap rejects an empty map (pointing at ExpressionCreator.emptyMap) and heterogeneous key or value types with IllegalArgumentException, matching the direction of Replace assert-based validation with real exceptions in :core and :isthmus #1047.

Isthmus still cannot convert nested structs or maps to and from Calcite; that follows in a stacked PR, which closes #375.

Part of #375

BREAKING CHANGE: ExpressionVisitor gains visit(Expression.NestedMap). Direct implementors must add it; implementors extending AbstractExpressionVisitor inherit the visitFallback default and need no change.

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