feat(coverage): add elisp backend

First of the pluggable coverage backends. Registers itself with coverage-core on load.

- :name is elisp
- :detect returns non-nil when the project root has a Makefile, Eask, or Cask alongside .el files at root or under modules/. The heuristic is deliberately loose. For anything unusual, .dir-locals.el can pin the backend with cj/coverage-backend.
- :run invokes make coverage in a compilation buffer. On success the callback fires with the LCOV path. On failure the buffer stays visible so the user can read the error.
- :lcov-path resolves to <project-root>/.coverage/lcov.info.

undercover is declared via use-package with :defer t so it's installed but not loaded at Emacs startup. The make coverage target will require it explicitly.

Tests cover Normal (Makefile + modules/, Eask + root .el, Cask + modules/), Boundary (no build file, Makefile without .el, empty directory), and Error (nonexistent root returns nil). The registration-on-load case is also verified.

The Makefile coverage target and the cj/coverage-report user command arrive in follow-up commits.

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