Fix Cap’n Proto maps with scalar and enum values

[lixin-wei]

Serializing a struct containing std::map<std::string, int> (or an unordered map with scalar values) fails during schema generation: Map(Int32) uses a scalar as a Cap’n Proto generic argument, which must be a pointer type. Enum-valued maps fail for the same reason, even when the map is empty.

Generate concrete map structs for scalar and enum values, with the value stored directly in each entry. Retain the existing generic Map(Value) schema for pointer-valued maps, so their generated schema and reader/writer paths stay unchanged.

Regression coverage checks empty and populated ordered/unordered maps, signed/unsigned 64-bit boundaries, booleans, floating-point values, enums, literals, and mixed/nested maps alongside string/list/optional values.

Validation: Release build with Clang and Cap’n Proto 1.1.0; all 59 Cap’n Proto CTest checks passed (58 runtime tests plus the compile regression).