serviceability: remove InterfaceV3 entirely

[elitegreg]

Part of the forward-compatible Device interfaces refactor.
Design: https://gist.github.com/elitegreg/e1d27c97034656a980fa8d7628ae16c5 (Design #8)

Depends on #3663 (rename).

Scope

InterfaceV3 was added by an earlier change, never deployed, and reverted in #3653. With the rename in the prior issue landed and all callers off the V3 path, the V3 type and its on-disk discriminant slot become dead weight.

Tasks

• Delete the InterfaceV3 struct.
• Remove the InterfaceDeprecated::V3 variant and discriminant 3.
• Delete into_v3() helper.
• Remove V3-related From / TryFrom / Validate impls.
• Remove any match arms that reference V3.
• No on-disk format change. The discriminant slot just becomes unused.

Tests

• Confirm no test references InterfaceV3 or discriminant 3.
• Workspace test suite passes.