Design: `*.test.ptk` self-contained test format + `test` command

[zharinov]

One file holds a source snippet, a query expression, and the expected result (separated by fences or a syntax extension). It auto-updates insta-style with --accept.

A separate fixtures+snapshots layout was considered and rejected:

• Per-file keying doesn't fit the workspace merge model.
• Parameterized refs aren't runnable by name.
• Root-anchoring problem → Design: trampoline — match a definition at any node #431.

Needs format design before implementation. test is already reserved in the shebang subcommand vocabulary. The CI story is check + test + batch run.