test: snapshot-style testdata layout; underscore-prefixed fragments; example CI job

[mpyw]

Reorganizes the test fixtures and wires the example module into CI.

• e2e_test.go → snapshot_test.go (TestE2E → TestSnapshots): it is a snapshot suite, not an end-to-end scenario.
• Flatten testdata/e2e/ into testdata/: the template is testdata/all_in_one.sql; its reusable fragments are underscore-prefixed by convention (testdata/_all_in_one.scope.sql, _all_in_one.active.sql); every generated artifact (all_in_one.expanded.sql, all_in_one.<case>.{prepared,embedded}.sql) lives under testdata/snapshots/.
• Same underscore-prefix convention in the example module (sql/users/_scope.sql, _active.sql; sql/audit_logs/_window.sql, _since.sql), dropping the fragment/ subdirectories. The embed becomes //go:embed all:sql so the _-prefixed files are included.
• CI: a separate example job (mise run example) that builds/vets/lints/deadcode/tests the example module. It is deliberately excluded from coverage, hence a separate job from check/cover; a mise run example task is added for local use.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (5ec23dc) to head (623775f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files          11       11           
  Lines         951      951           
=======================================
  Hits          918      918           
  Misses         21       21           
  Partials       12       12           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.