[Core] Complete unit test suite for VecId

[alxbilger]

Gemini 3.7 Flash generated the tests. I had to add the method getGroup in TStandardVec<V_ALL, vaccess>, otherwise it's ambiguous and it does not compile.

[with-all-tests]

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[fredroy]

"This compares the implementation to itself and would pass if getName were completely wrong. Hard-code the expected strings."

[fredroy]

ConstVecId are not tested and no V_ALL neither

[fredroy]

Suggested change

	EXPECT_EQ(coordTargetWrite.getType(), VecType::V_COORD);

[fredroy]

"This (and the 4 other) checks only the index on the V_ALL→specific path"
So we should check also the resulting type

[fredroy]

Suggested change

	EXPECT_EQ(coordTargetRead.getType(), VecType::V_COORD);

[fredroy]

Suggested change

	EXPECT_EQ(coordTargetRead.getType(), VecType::V_COORD);

[fredroy]

Suggested change

	EXPECT_EQ(derivTargetWrite.getIndex(), 23u);
	EXPECT_EQ(derivTargetWrite.getIndex(), 23u);
	EXPECT_EQ(derivTargetWrite.getType(), VecType::V_DERIV);

[fredroy]

Suggested change

	EXPECT_EQ(matDerivTargetWrite.getIndex(), 24u);
	EXPECT_EQ(matDerivTargetWrite.getIndex(), 24u);
	EXPECT_EQ(matDerivTargetWrite.getType(), VecType::V_MATDERIV);

[fredroy]

Suggested change

	}
	// Access: write-access ids convert implicitly to read-access ids.
	static_assert(std::is_convertible_v<VecCoordId, ConstVecCoordId>);
	static_assert(std::is_convertible_v<VecDerivId, ConstVecDerivId>);
	static_assert(std::is_convertible_v<MatrixDerivId, ConstMatrixDerivId>);
	// A specific id converts implicitly to the generic V_ALL id.
	static_assert(std::is_convertible_v<VecCoordId, VecId>);
	static_assert(std::is_convertible_v<VecCoordId, ConstVecId>);
	// The reverse narrowing must stay explicit: constructible, never implicit.
	static_assert(!std::is_convertible_v<VecId, VecCoordId>);
	static_assert(std::is_constructible_v<VecCoordId, VecId>);
	// Unrelated specific types never interconvert.
	static_assert(!std::is_constructible_v<VecCoordId, VecDerivId>);
	static_assert(!std::is_assignable_v<VecCoordId&, VecDerivId>);
	// Assignment mirrors construction, including the V_ALL -> specific direction.
	static_assert(std::is_assignable_v<VecCoordId&, VecId>);
	static_assert(std::is_assignable_v<VecId&, VecCoordId>);
	}

[fredroy]

(Could be somewhere else)
"The suite tests values but never the type system, which is what TVecId mostly is"