checker, cgen: fix fixed array struct initialization

[medvednikov]

Summary

• use the empty-struct initializer sentinel for direct fixed-array aliases of empty structs
• reject references assigned to non-pointer fixed-array struct fields before C generation
• mirror the checker fix in v3 and cover both fixed-array cases there
• add regression coverage for both cases

Fixes #27894
Fixes #27957

Tests

• ./vnew vlib/v/tests/aliases/alias_fixed_array_of_struct_test.v
• ./vnew -cc tcc vlib/v/tests/aliases/alias_fixed_array_of_struct_test.v
• ./vnew -silent vlib/v/compiler_errors_test.v
• ./vnew -silent test vlib/v/checker/
• ./vnew -silent vlib/v/slow_tests/inout/compiler_test.v
• ./vnew -silent vlib/v/gen/c/coutput_test.v
• ./vnew -silent vlib/v3/tests/fixed_array_local_zero_init_codegen_test.v

The full vlib/v/ suite was skipped as requested.

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