strip_src_prefix config: support flat layouts where the import package is literally src

[axisrow]

Problem

Projects whose import package is literally src (flat layout: src/__init__.py, imports like from src.calc import ...) cannot run mutmut 3.8 at all — stats collection dies during the first trampoline hit:

AssertionError: Failed trampoline hit. Module name starts with `src.`, which is invalid

The root cause is that mutant naming assumes the src-layout convention, where src/ is only a container directory:

1. get_mutant_name (utils/format_utils.py) unconditionally strips a src. prefix. For a flat layout the stripped name (collection_queue.x_foo__mutmut_1) no longer matches the imported module (src.collection_queue).
2. That breaks the trampoline routing in mutation/trampoline.py (module != decorated_func.__module__ never matches → mutants silently never apply), the stats mapping (tests_by_mangled_function_name lookups miss), and finally record_trampoline_hit (mutation/stats.py) asserts on the src.-prefixed recorded name — killing the whole stats run.

Sandbox importability itself already works for flat layouts (setup_source_paths puts mutants/ on sys.path, so from src... resolves to the mutated copy) — only the name shaping is broken.

Fix

New config [tool.mutmut] strip_src_prefix (default true = current behavior, so src-layout projects are unaffected):

• false: get_mutant_name keeps the full src.-prefixed module path — routing, stats keys, and the assert all become consistent;
• record_trampoline_hit asserts only while stripping is active, and its message now points flat-layout users at the knob;
• unit tests for the config key and both get_mutant_name modes;
• new e2e fixture e2e_projects/flat_src (src as the import package) with a result snapshot: 11 mutants generated with src.-prefixed names, 6 killed / 5 survived — i.e. mutants verifiably apply and route, while boundary-condition mutants (clamp at value == low) survive exactly as the fixture's tests intend.

Verification

• New e2e snapshot test passes; full local suite: 456 passed. The 3 pre-existing failures on my machine (test_e2e_type_checking ×2 — type-checker version drift, test_safe_setproctitle — macOS CoreFoundation fork) reproduce on a clean checkout without this patch.
• Verified end-to-end on a real flat-layout project (~1.8k-file Telegram engine, from src.* imports throughout): with the knob, stats collection completes for the first time and mutants execute; stock 3.8 asserts out at the same point.

🤖 Generated with Claude Code

[boxed]

Projects that do this are broken. Please fix the project. I've closed these issues before, please search closed issues before submitting.