Pins the library cases' send types

[johnnyt]

Refs: st-sm5s

What this adds

conformance/README.md's library-world section says every library case
whose chart sends registers both send types, library:timer then
library:route, and that a case whose chart sends nothing carries no host
object. No test read host.send_types: the existing host test in
test/corpus/corpus_files_test.exs asserts that a case whose document sends
carries a host object, and nothing checked what that object registers. A
library case registering only library:timer, or the two types in the other
order, passed every test.

The new test in the same file reads every committed case JSON under
conformance/cases/library/, accepts/ and diff/ (the three directories
the README places in the library world), keeps those with a host object,
and asserts that each carries send_types exactly
["library:timer", "library:route"], naming any case that does not. It
fails when it finds no such case, so it cannot pass on nothing. The cases
under send/ and system_variables/ keep their own domain, as the README
says, and are not read. A library case added later is covered without a
change here.

The sabotage checks

Each in conformance/cases/library/loan_lost_after_timer.json, the file
copied first:

• the two send types swapped: the test fails on its assertion, naming the
  case and ["library:route", "library:timer"];
• library:route dropped: the test fails on its assertion, naming the case
  and ["library:timer"].

After each, the file was restored from the copy and confirmed byte-equal
(cmp, and git diff --exit-code), and the test passed again. The test
asserts committed case files and no lib/ behaviour, so its # sabotage:
line states the exemption and names these two mutations.

Test only; no changelog fragment (changelog.d/README.md excludes the test
harness and conformance fixtures).

Gate

Full mix quality (mix gate.verify) green on this commit's tree after the
rebase onto the current main, and the merge profile
(mix quality --profile merge) green, the ADR judge skipped because no file
is in a judged scope.

Review

Tier: gate (a test over committed fixtures). The in-turn review of the diff
found nothing to change.