irmin: merlint 0/69, switch fuzz to alcobar, add real tests
- Switch fuzz suite from crowbar to alcobar (supports --gen-corpus)
- Remove fuzz_common.ml / gen_corpus.ml (inline truncate helper)
- Add fuzz .mli files exporting only suite
- Rename test_tar → test_irmin_tar to match library module name
- Export Irmin.Worktree, expose index_entry/index types
- Add real heap tests (13), worktree tests (5)
- 37 unit + 10 fuzz = 47 tests, merlint 0/69 clean