`wvr module test`: before/after fixture harness for external module repos + reusable CI workflow

[Max-Levitskiy]

Priority: P2 · Epic: #82

Why

crates/cli/tests/examples_suite.rs proves primitives inside this repo, but a module in
another repository has no way to run the same before/after snapshot discipline against
itself. Module authors need "apply on before/ equals after/, check on after/
exits 0, check on before/ exits 2" as a one-command test.

What

1. wvr module test [<module-dir>] [--case <name>] [--update]: for each
   tests/cases/<name>/ with before/, after/, and weaver.yaml (consumer config
   whose module source: is the module under test), copy before/ to a temp dir, run
   check (expect exit 2 unless expect_pass: true in case.yaml), run apply, diff
   against after/ (ignore weaver.lock, .rw/), run check again (expect 0), run
   plan --detailed-exitcode (expect 0). --update rewrites after/.
2. The examples harness in this repo reuses the same core so behaviour cannot drift.
3. Reusable workflow .github/workflows/module-ci.yml (workflow_call): installs wvr
   (pinned), runs wvr module validate and wvr module test, uploads the JSON
   reports. Module repos call it with three lines.

Acceptance

• The scaffold from wvr module new scaffold, module authoring guide, and self-describing manifest #83 passes wvr module test in a fresh clone.
• A test in this repo runs wvr module test against a fixture module with one failing
  case and asserts the diff output names the file.

Depends on

#83, #77, #73.