Adds a test keeping new tracker ids out of lib/

[johnnyt]

What

A new test, StatifierExamples.TrackerIdTest, refuses a new tracker id in any file under lib/. A tracker id is a family tracker prefix, a dash and three or four base36 characters; in a moduledoc or a comment it points a reader at an issue instead of saying what the code does.

lib/ already carries such ids, so the test works the way StatifierExamples.PrivateIdTest does: a committed baseline, test/fixtures/tracker_id_baseline.txt, counts the ids each lib/ file carries at this commit. A count above the baseline fails, naming the file and the id; a count below it fails too, naming the baseline line to lower or delete, so an id that leaves a file cannot come back unseen. CSS class names of the same shape are excluded by name (sr-only); longer class prefixes such as sb-editor never match the shape. The test globs lib/ at run time, so a file added later is covered the moment it exists.

Provenance

• The bead read lib/ as already free of tracker ids. It is not: the ids are in moduledocs and comments across lib/statifier_examples/ and lib/statifier_examples_web/ (the baseline lists every one). A test that forbids every id outright would be red at the tip, so this PR takes the baseline shape of private_id_test.exs instead, chosen in this PR as a test-shape choice. Rewriting the existing ids into their substance is left as a follow-up; each rewrite lowers a baseline line, and the second assertion makes that lowering required.
• No lib/ file changes in this PR.

Checks

• Sabotage, each run and reverted from a copy: an invented id planted in charts/fixture.ex's moduledoc, a second copy of an id documents.ex already carries, and the id removed from charts/fan_out.ex's moduledoc each turned "no lib/ file gains a tracker id" red on its assertion; dropping only from the excluded class words and shortening the id shape to three characters each turned the pattern test red on its assertion. The one-line notes sit above each test.
• Full mix quality green on the committed tree: format, compile, credo, dialyzer, deps, and the full suite with coverage.

In-turn review

I re-read the diff against the bead: its acceptance asks for a test that fails when a tracker-id shape appears in a lib/ moduledoc or comment, CSS classes allowed, sabotaged with an injected id and green at the tip. The planted-id sabotage above is the injected id; the sr-only and sb-editor negatives in the pattern test are the CSS allowance; the suite is green at the tip. I checked the prefix list against every family tracker's prefix, and checked every non-id match of the shape in lib/ today (there is one, sr-only in core_components.ex). The moduledoc's "What it does not catch" list names the known gaps: a prefix not yet listed, an id spelling an excluded class word, and a baseline line added beside a new id.