tests: add fat closure acceptance targets

[MartenH]

This is a small draft/test-target PR for the fat-closure direction discussed around #27446/#27445.

It intentionally does not carry the frame-epoch reclamation runtime. Instead it adds opt-in acceptance tests for the desired representation boundary:

• under -d fat_closures, pure V stored closures should be GC-visible values and should not emit builtin.closure trampoline creation/init code;
• explicit closure-to-raw-function-pointer coercions still have a tested boundary where a C-callable shim is expected.

Normal test runs pass because the checks are gated behind fat_closures. To exercise the target today:

v -d fat_closures -gc none test vlib/v/tests/fns/fat_closure_acceptance_test.v

That currently fails on the pure-V closure assertion, as intended: it is the executable acceptance target for a future fat-closure implementation.

[MartenH]

After doing more research this is quite and undertaking and consider all the changes in memory management I decided to close at this tine