Close three pre-relaunch gates: pytest scope, licensing catch-all, conformance attestation

[rocklambros]

Three pre-relaunch gates, each independently revertable.

Scope pytest collection to tests/. The site build depends on a bare uv run pytest -v running in the environment uv.lock produces. With no testpaths, pytest collects the whole repository, so a suite whose dependencies sit outside the lockfile fails at collection and takes the deploy with it. PR #22 adds 23 adapter test files needing rfc8785, nvidia-nat-core, and ruamel, none of them locked. Measured against a real merged tree:

pyproject.toml	Collection
before	206 collected, 22 errors, interrupted
after	195 collected, 0 errors

Interrupted collection exits nonzero, which fails the test job, which stops the build job that declares needs: test. On main as it stands today the change is inert: 194 passed and 1 skipped both with and without it, because nothing currently lives outside tests/.

CONTRIBUTING now says where guards belong and why a test written elsewhere never runs. That silence is what let two contributors build suites CI would not have executed.

Give LICENSING.md a catch-all row. The table mapped ten paths and stopped, so a new top-level directory arrived governed by nothing until someone noticed. adapters/ is the next one due. Apache 2.0 is the default because a new directory is usually code, and an over-permissive grant on prose costs less than a ShareAlike obligation attaching by accident to reference code that adopters copy into their own systems. A prose directory still wants its own explicit CC-BY-SA-4.0 row.

Say who verifies an ACS-Core conformance claim. Nobody does, in v0.1.0. profiles_supported and profiles_accepted are self-declaration on the wire, and this release ships no conformance suite, no registry, and no steward to arbitrate a disputed claim. The page described what the label guarantees without saying the label is the implementer's own assertion, which is the gap issue #19 raised. The paragraph sits below the ACS-Core requirement list rather than inside it, so the conformance.md line numbers PR #22's citation guard pins do not move.

Closes part of #19.