Validate the macOS arm64 binary

[ivanvyd]

RELEASING.md and docs/compatibility.md both record the macOS arm64 binary as built, not
tested. It is produced by every release and has never been run.

Nobody on the project has a Mac, which is the entire reason this is open.

What closing this looks like

Download lakespeak-<version>-osx-arm64.zip from a
release, then:

# confirm the download is genuine first
sha256sum -c --ignore-missing SHA256SUMS.txt

unzip lakespeak-0.1.0-osx-arm64.zip
./lakespeak --version
./lakespeak --help

If you have a Databricks workspace with a Genie Agent, lakespeak auth check and
lakespeak agents list exercise the parts most likely to differ by platform — process invocation
of the Databricks CLI, and terminal rendering.

Then update the Platforms table in docs/compatibility.md with the macOS version, the chip and
the date. Gatekeeper may quarantine an unsigned binary; if it does, that is worth recording too,
because it affects everyone on macOS and is currently undocumented.