row 88: the suite steps build the example cdylibs their tests load by name, always (stacked on #831)

[MattJackson]

Row 88 (predev test green). This is a stacked draft on #831 (lane-ci-test-gated) and stays a draft until #831 lands (rule 19).

Root cause

A test that dlopens an example cdylib fails, and never skips, when that library is absent (crates/busbar/tests/common/plugins.rs::missing). The test step only got busbar-plugin-loader's examples because a workspace-wide test build happened to build them. Under scope: affected, a run of busbar alone builds no other package's examples. On #833, a run of 1,189 tests covering only busbar, that left neutral_frame_door unbuilt, and 22 tests failed with "the neutral_frame_door cdylib is not built": root::serve::money_tests (serve_money.rs), root::serve::framed_tests and two root::registry::tests. The same tests pass on full runs (#842, #821).

Change

• c2c3f548b0 RED: xtask/tests/ci_test_fixtures.rs::an_affected_scope_run_of_busbar_alone_builds_the_examples_its_tests_load.
  • It collects every package a test under crates/ names in its fixture-build command (cargo build -p <package> --examples). Today that is busbar-plugin-loader and busbar. The test fails if the scan finds neither, so it can never pass on an empty scan.
  • It then requires every .ci/*.yaml step that runs cargo nextest run to run cargo build --locked -p <package> --examples for each of them before its first suite.
  • It is red on this commit for default.yaml:test, default.yaml:test-gated and predev.yaml:test-gated.
• f6ada72804 fix: those three steps build both packages' examples by name, unconditionally. The build cannot be limited to scopes that need it, because no variable gives a step the affected set (.ci/ uses only MODERNCI_TO_SHA, MODERNCI_TO, MODERNCI_COMMIT and MODERNCI_ONLY). Building by package name also keeps the build from being narrowed by a scope that rewrites --workspace.

Not a $ change.