[miguelcoba]

Closes #1.

Problem

For a single (non-umbrella) app, Mutare.Sandbox wraps only test/test_helper.exs. A project that sets test_paths: in its mix.exs, for example ["test/unit", "test/integration"], loads those directories' helpers instead, so the coverage probe never runs: it ends with coverage dump unreadable (:enoent), and every covered mutant runs the whole suite. Kills stay correct; only test selection is lost, which on a large suite makes a run far slower.

Solution

• New option test_paths: in .mutare.exs, mirroring test_paths: in the target's mix.exs. It defaults to ["test"], so nothing changes unless it's set. Mutare reads none of the target's Mix config, so the option mirrors the setting rather than evaluating mix.exs.
• Sandbox.helper_rels/3 wraps <path>/test_helper.exs for every entry. The umbrella clause is unchanged.
• Several helpers in one BEAM reuse the safeguards umbrellas already rely on: the ETS tables are created only if missing, the bootstrap keeps its first evaluation, and each after_suite writes the same dump.
• Docs: the .mutare.exs reference in mix help mutare, a troubleshooting entry for coverage dump unreadable (:enoent), and the CHANGELOG.

Verification

• New sandbox test: with two configured test paths, both helpers are wrapped and no test/test_helper.exs is created.
• New end-to-end test in coverage_runner_test: a project with two test_paths: entries gets selection == :tests with the option. Without it (checked once, not committed), the same project gives selection == :suite.
• Full suite: passes apart from 6 tests that fail the same way on an untouched master on the same machine (CompilerOptionsIntegrationTest ×3, NamespaceRunnerTest, UmbrellaTest, SwapRedundancyTest). Format and Credo are clean; Dialyzer reports the same harvest.ex warning and Dialyxir crash as master.
• On a project with 10 test_paths: entries, using this branch as a git dependency, every covered mutant gets per-test selection instead of a whole-suite run.