Gate the Ruby version the README counts at, and drop a limitation that is gone - #889

…t is gone

This branch started on a false premise, which is worth stating because the
correction is the interesting part. I set out to gate the README's conformance
total, believing nothing checked it, and wired a CI job around the published
`conformancefigure`. Then I read
`TestReadmeConformanceCountMatchesTheBaseline`, which has been checking exactly
that for weeks — against BASELINE in the same tree, with no network and no
pinned external version. My job would have duplicated a better check and added
a dependency. It is not in this commit.

What that test does *not* cover is the version the total is counted at, and
that version selects the population counted: the move from 3.4.1 to 4.0.9
changed which `ruby_version_is` guards admit examples (+144 passing, 27 gone
red). The same number means a different thing at a different version.

TestReadmeRubyVersionMatchesTheRuntime reads `rubyVersion` from
internal/vm/kernel_introspect.go and checks it two ways:

* forward — every present-tense claim ("rbgo reports `"4.0.9"`", "already
  reflect 4.0.9") must equal it, and fewer matches than shapes is a failure, so
  a rewording that escapes the patterns fails instead of silently checking less;
* inverse — a line mentioning RUBY_VERSION and naming a different version must
  declare itself historical ("sweep", "the move from"), and the exemptions are
  logged rather than applied silently.

Both halves are needed, which a deliberate break showed. With the table row set
to 4.0.8 the forward check named it while the inverse check EXEMPTED the same
4.0.8, because that line also says "the move from": a line-granular marker
meant for the version left behind covered a wrong present-tense claim sitting
beside it.

Two README corrections, both measured rather than read:

* line 16 said "It **runs** 23,672". 23,672 is what passes; rbgo runs
  considerably more. The figure was right and the verb was not, which is why
  the existing numeric gate was green the whole time — no check on a number can
  see a wrong verb.
* *"A backtick command holds the interpreter lock"* no longer reproduces. The
  probe the README carried printed `frozen` in October and prints `ran` now,
  five runs out of five, and rubydiff reports the program AGREE with MRI 4.0.7.
  Removed, with no cause attributed: naming one without a control would be a
  guess dressed as a fact.

`Timeout.timeout` was re-checked on the same pass and still fails open — rbgo
prints `returned` then `nil`, MRI 4.0.7 prints `raised` then `0.3` — so it
stays, now with that measurement beside it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>