Moves the dev dependency to ex_quality 0.15.1

[johnnyt]

Moves the dev-only ex_quality dependency from 0.15.0 to 0.15.1 (bead sui-64yg).

Why

ex_quality 0.15.1 fixes the Docs stage. 0.15.0 counted only ExDoc warning lines that start with warning:; ExDoc 0.40 on Elixir 1.18 indents them, so the stage could report no warnings over docs that did warn. 0.15.1 reads both shapes and passes --warnings-as-errors to mix docs.

What changed

• mix.lock: the ex_quality line only (0.15.0 to 0.15.1, both hashes). No other lock line moved.
• mix.exs is unchanged: its constraint ~> 0.15 already admits 0.15.1.
• No docs change: the fixed Docs stage reports no warnings on this repo, so there was nothing to fix. The pre-existing skip_undefined_reference_warnings_on: ["CHANGELOG.md"] stays as it was; no skip was added.
• No changelog fragment: changelog.d/README.md excludes quality gate and tooling changes.
• No version bump.

Verification

• mix deps.update ex_quality resolved 0.15.1 from Hex.
• MIX_ENV=dev mix docs --warnings-as-errors exits 0 with no warning lines.
• Full mix quality green with the Docs stage on 0.15.1 ("Docs: No warnings"), and mix quality.verify attests a full run (scope all, no profile). Gettext and Sobelow are the repo's two declared not-applicable skips.
• The stage discriminates: a reference to an undefined function on a real module, appended to a guide and then restored from a copy, turned the Docs stage red with one warning at that line. (A reference to a module that does not exist does not warn at all; that is ExDoc's own behaviour, not the stage's.)

Review

Tier: gate. The diff is one mix.lock line for a dev-only dependency; it touches no public function, module, option, callback, wire shape or fixture, and is far under the size threshold. In-turn review: I re-read the diff against the bead's acceptance. The lock resolves 0.15.1 from Hex under an admitting constraint, and only the ex_quality line moved. The full gate is green with the fixed stage. mix docs --warnings-as-errors exits 0, so there were no warnings to fix and no skip list was needed. This repo's CLAUDE.md asks for no gate-change log entry or record for a tooling change. There is no version bump. No record under docs/adr/ is touched.