Test Subprojects: bb_example_wx200 dialyzer fails on every source-changing PR

[jimsynz]

The bb_example_wx200 job in Test Subprojects fails its dialyzer step on
every pull request that touches bb's source, and passes on every push to
main. The failure is not a type error — dialyzer analyses zero modules:

:dialyzer.run error: File not found:
  /home/runner/work/bb/bb/bb_example_wx200/_build/dev/lib/bb_example_wx200/consolidated/Elixir.Collectable.beam
...
  files: []
:dialyzer.run error: Analysis failed with error:
No .beam files to analyze (no --src specified?)
Halting VM with exit status 1

Every other tool in that job passes — compiler, credo, ex_doc, ex_unit,
formatter, gettext, mix_audit, reuse, unused_deps.

It correlates with the branch, not the change

Recent Test Subprojects runs, bb_example_wx200 job only:

Run	Branch	Event	Result
32674375855	fix/normalise-parameter-units	pull_request	failure
32619948357	v0.30.1	push (tag)	failure
32619948241	main	push	success
32619919149	main	push	success
32619704733	fix/compile-time-unit-error-messages	pull_request	failure
32613985655	main	push	success
32613733359	dependabot/hex/dev-dependencies-…	pull_request	success
32613651172	renovate/beam-bots-.github-digest	pull_request	success

fix/compile-time-unit-error-messages is the branch that became 62ce89e and
shipped as 0.30.1. It failed as a PR, then passed on main with the same code.
The two dependency-only PRs passed. The failure log is byte-identical across
all three failing runs.

So the discriminator is whether the run recompiles bb into the subproject's
_build, not whether the change is sound.

Likely mechanism

The _build cache key is

key: …-${{ matrix.project.name }}-build-${{ hashFiles(format('{0}/mix.lock', matrix.project.name), 'bb/mix.lock') }}

which does not move when bb/lib/** changes — by design, since BB_VERSION=local
makes bb a path dep. The Force a PLT re-check step then removes
_build/*/*.plt.hash so dialyxir re-checks against bb's newest exports.

On a source-changing branch that combination appears to leave
_build/dev/lib/bb_example_wx200 without its consolidated protocol beams, and
dialyxir's file list comes back empty rather than triggering a rebuild.
bb_example_wx200 sets consolidate_protocols: Mix.env() == :prod, so nothing
regenerates consolidated/ in :dev, and the other 18 subprojects are
unaffected.

Why it matters

The downstream matrix is the main safety net for a breaking change to core —
it is the reason a fix! to BB.Parameter can be reviewed with confidence.
A job that reliably goes red for reasons unrelated to the diff trains everyone
to skim past it, which is exactly when a real downstream break gets waved
through.

Worth considering

• Include bb's source in the _build cache key for subprojects, or drop
  _build/dev rather than just the PLT hash, so a source change rebuilds
  rather than half-restores.
• Or have the step force the :dev build explicitly (MIX_ENV=dev mix compile)
  before mix check, so dialyzer always has beams to analyse.