ci(dialyzer): cache PLT and split build/analyze steps

[davydog187]

Cache the Dialyzer PLT in CI to cut job time from ~3m30s to <60s

Plan: .agents/plans/A38-dialyzer-plt-cache.md

Goal

Cache the Dialyzer PLT files in GitHub Actions so the dialyzer job runs in
under 60 seconds on cache hits instead of ~3m30s every run.

Root cause

The dialyzer job had no PLT cache and no stable PLT path. Every run
rebuilt the entire OTP/Elixir/ex_unit PLT from scratch (~2m45s), then
ran analysis on top of that (~15s). The PLT build cost was paid even
when nothing changed.

Success criteria

• mix format --check-formatted passes
• mix compile --warnings-as-errors passes
• mix test passes — 55 doctests, 51 properties, 1771 tests, 0 failures
• CI YAML is valid (manually reviewed)
• On first CI run (cache miss): dialyzer job completes successfully (~3 min, same as today)
• On second CI run (cache hit): mix dialyzer step < 60s

Changes

 .github/workflows/ci.yml | 12 ++++++++++++
 .gitignore               |  3 +++
 mix.exs                  |  4 +++-
 3 files changed, 18 insertions(+), 1 deletion(-)

mix.exs — pin plt_core_path/plt_local_path to priv/plts/ so the
PLT files land in a deterministic, cache-able directory. Also drop :mix
from plt_add_apps — lib/ has no runtime references to Mix.* (only
tasks/ does, and that path is excluded from the production
elixirc_paths/1).

.gitignore — exclude /priv/plts/ so PLT build artefacts are never committed.

.github/workflows/ci.yml — in the dialyzer job:

• Add Restore PLT cache step (actions/cache@v4) immediately after
  install-deps. Cache key includes OS, Elixir version, OTP version, and
  mix.lock hash. The restore-keys fallback means a mix.lock bump
  triggers an incremental PLT update instead of a full rebuild.
• Split the single mix dialyzer step into Build PLT (mix dialyzer --plt)
  and Run dialyzer (mix dialyzer) so CI logs show build vs. analysis
  time separately.

Verification

mix format --check-formatted    # ✓ clean
mix compile --warnings-as-errors # ✓ no warnings
mix test                         # ✓ 1771 tests, 0 failures, 30 skipped

Out of scope (intentional)

• Changing what dialyzer checks (no new ignores, no flag changes)
• Moving dialyzer off the merge-gating path
• Enabling dialyzer on the OTP 29 / Elixir 1.20 matrix row
• Any change to mix test or other CI jobs