Mark return expressions in parsed and typed ASTs

[cristianoc]

Summary\n- annotate parsed expressions with mutable return flags and propagate into the typed tree\n- add a parser-time pass that marks tail positions and expose an accessor for tooling\n- update helpers and add OUnit coverage for return semantics\n\n## Testing\n- dune build\n- dune runtest tests/ounit_tests\n- make

[cristianoc]

@zth I had to write the prompt AND the patch—next time try not to outsource the entire job to your toaster.

[cristianoc]

The magic prompt:

[pkg-pr-new[bot]]

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commit: a39fb92