Make standard-library updates version-aware and transactional

[LunaStev]

Problem

A compiler that rejects retired syntax can still load an older installed standard library containing that syntax. The resulting parse failure may look like a user-source error. The current update flow also removes the installed std before the replacement has been fetched and validated.

Scope

• Add language or compiler compatibility metadata to std/manifest.json.
• Validate compatibility before compiling an imported std module.
• Stage std updates in a temporary directory.
• Validate the manifest and Wave source corpus before replacing the installed copy.
• Atomically replace the installed std only after validation succeeds.
• Preserve the previous installation when download or validation fails.
• Produce a direct diagnostic with the installed and required compatibility revisions.
• Record enough install metadata to identify the source revision.

Completion criteria

• An incompatible installed std fails with an explicit update instruction.
• A failed update leaves the previous std usable.
• Successful updates never expose a partially copied tree.
• Tests use isolated temporary home directories.
• Diagnostics identify the imported std file rather than blaming the entry source.