build: switch Makefile to eask, wire up undercover coverage

I wanted a coverage number, so I added an Eask file declaring the runtime dep (xterm-color) plus three dev deps (undercover, package-lint, elisp-lint). The Makefile now runs every test and lint recipe through `eask emacs`. That drops the hand-rolled `(require 'package)` + `add-to-list 'package-archives` boilerplate that was duplicated across six recipes.

I added a `make deps` target that runs `eask install-deps --dev`. I also added a `make coverage` target that loads `tests/run-coverage-file.el` before each unit-test file. Undercover instruments the three source files first, then the test loads pick up the instrumented copy. Per-file results merge into `.coverage/simplecov.json` in simplecov format.

I expanded `validate-parens` and `compile` to cover all three source files instead of just `wttrin.el`. Lint stays scoped to the main file for now.

Coverage right now is 84% overall: wttrin.el 92%, wttrin-geolocation.el 100%, wttrin-debug.el 27%. The debug module is low because only the integration test exercises it. The coverage loop runs unit tests only.

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