Builds the documentation with mix docs and fails on any ExDoc warning.

ExDoc warns about real defects a reader will hit - a reference to a function that does not exist, a moduledoc link that resolves nowhere, an undefined anchor - and mix docs exits 0 anyway on most versions. Warnings that fail no build accumulate, so this stage is the ratchet: a project that reaches zero warnings stays there.

✓ Docs: No warnings (2.1s)
✗ Docs: 3 warnings (1.9s)

Each warning becomes a finding at the file:line ExDoc reports:

lib/my_app/user.ex
  42  [error] documentation references function MyApp.User.fetch/2 but it is undefined or private (ex_doc)

A warning ExDoc reports without a location falls back to the tool's output verbatim, so a warning is never hidden behind a parse.

Opt-in

Unlike the other tool-backed stages this one is off by default, even when :ex_doc is installed. Nearly every published package depends on :ex_doc to build its docs, so enabling on detection would turn currently-green gates red on upgrade, and this project does not move anyone's gate by default. Enable it in .quality.exs:

docs: [enabled: :auto]   # on when :ex_doc is installed (recommended)
docs: [enabled: true]    # forced; errors if :ex_doc is missing

With enabled: :auto a project without :ex_doc reports the stage as skipped (:ex_doc not installed), the way doctor and sobelow do.

What it builds

mix docs --formatter html --output <tmp dir>: one formatter, because the epub build repeats the html build's warnings, and a temporary output directory that is deleted after the run, because a checker that leaves a doc/ tree behind has written to the repository. The project's own mix docs output is untouched.