Compat baseline 4/5: test representative plugin startup and feature paths against the minimum and latest host #134

Effort: M

Part of #102. Depends on Compat baseline 1/5 and 2/5.

The matrix is only a claim until something exercises it. Add tests that run representative plugin startup and feature paths against the declared minimum host and the latest host.

Scope

• Pick a small representative set (e.g. Piano and Splitscreen basic paths, a plugin using each optional API from the matrix).
• Run startup and the key feature path against both the minimum-baseline commit and current main, in CI or a documented repeatable script.
• Record the tested host and plugin revisions back into the compatibility matrix.

Acceptance criteria

• A test run fails if a plugin that claims support for the minimum host breaks on it.
• Tested revisions are recorded in the matrix, replacing "source-audited" with "tested" where applicable.
• The check does not need network access beyond cloning the pinned commits (or states what it needs).

Out of scope

Certifying every historical build.