test: migrate golden specs to Backspin.compare

[rsanheim]

Migrates the rspec-vs-plur golden specs to Backspin.compare (new in backspin 0.13.0) and deletes the workarounds that only existed because there was no differential-testing API.

57 insertions, 448 deletions. Specs alone: -272/+35.

The pattern being removed

Each of these specs called Backspin.run twice with the same record name: — once for rspec, once for plur — so a recorded snapshot acted as a hand-rolled differential test:

chdir(fixture) { Backspin.run(rspec_cmd, name: "x", filter: normalize) }
result = chdir(fixture) { Backspin.run(plur_cmd, name: "x", filter: normalize) }
expect(result.verified?).to be(true)

Backspin.compare does this directly: one call, one chdir, one filter, nothing persisted to disk.

Three workarounds fall out with it:

• The "args" => ["[SOMETHING_COMMAND]"] placeholder in every filter. compare never compares argv, so normalizing it was dead code.
• expect(result.verified?).to be(true). A mismatch raises Backspin::VerificationError with the diff before the expectation is reached, so the assertion could never fail — only fire after the raise already had.
• Three examples in single_failure_golden_spec.rb that differed only by record name and were otherwise byte-identical.

Six snapshot YAMLs are deleted. The remaining six in fixtures/backspin/ are ordinary single-command snapshot tests using Backspin.run correctly and are untouched.

Order-dependence fixed

minitest_integration_spec.rb recorded in one example and verified in a different one, so it passed only under RSpec's defined order and broke under --order random. One compare call makes that failure mode structurally impossible.

single_failure_spec.rb removed

It was a hand-rolled Backspin.compare: same two commands, same fixture, its own copy of the timing normalizer, asserting stdout_lines == rspec_stdout_lines. Strictly weaker than compare, which also checks exit status and stderr.

Its absolute assertions — exit status, failure count, plur's stderr banner — moved onto result.actual in the compare-based example, so they cost no extra subprocess pair. --color=never coverage remains in colorized_output_spec.rb and tty_output_spec.rb.

A pending spec dropped rather than migrated

The plur -n1 vs raw-grouped-minitest comparison could never pass: the RSpec-style duration line and consolidated progress dots are deliberate decisions pinned by passing specs in that same file, and the dot/puts interleaving position isn't stable because plur's worker command passes no seed. The real defect it pointed at — minitest dropping test stdout — is fixed separately in #106.

Verification

Full suite green in both defined and random order. Each migrated spec was confirmed to still fail when it should: pointing plur at --color=never against rspec's --force-color raises VerificationError with a real diff in all three golden specs.

Worth noting the migration made the suite faster, not slower. Backspin.run shells out in verify mode too, so the old code was already re-running rspec on every test run and comparing it against a snapshot of itself. Collapsing the duplicate examples removed 4 subprocess spawns per run.

Also bumps backspin to ~> 0.13.0 and regenerates the Appraisal gemfiles and lockfiles. Appraisal2 emits hashrocket syntax, so there is a follow-up standard:fix commit restoring the 1.9 hash syntax those files already used.