cgen: fix fixed array return via local

[medvednikov]

Summary

Fixes #27345.

• Copy fixed-array identifier returns from the raw local array instead of reading .ret_arr from it.
• Add a regression test for returning a fixed array through an intermediate local initialized from another fixed-array-returning function.

Root Cause

Fixed-array function returns use a wrapper struct with a .ret_arr field in generated C, but ordinary local fixed-array variables are raw C arrays. The return path was using the fixed-array return-wrapper type metadata to decide that an identifier return should read local.ret_arr, which is invalid for raw local arrays.

Validation

• ./v -g -keepc -o ./vnew cmd/v
• issue repro with ./vnew run /tmp/.../repro.v
• ./vnew -silent test vlib/v/tests/return_fixed_array_test.v
• ./vnew -silent vlib/v/gen/c/coutput_test.v