Fat Pointer Hardening: All 17 Journeys to 10/10

Fix 3 CRITICAL bugs and 1 HIGH codegen issue discovered by the fat pointer code journeys (J14-J17), then harden the compiler with a combinatorial test matrix that crosses fat pointer types with every language feature. The goal: all 17 code journeys score 10.0/10, and the test matrix prevents future regressions at any intersection of fat pointers with other features.