doc: update `inferInstanceAs` docstring and rename `normalizeInstance` to `wrapInstance`

This PR updates the `inferInstanceAs` docstring to reflect current behavior: it now
requires an expected type from context and should not be used as a simple `inferInstance`
synonym. The broken `#check inferInstanceAs (Inhabited Nat)` example is replaced with a
working one that demonstrates the intended transport use case.

Additionally, rename `InstanceNormalForm.lean` to `WrapInstance.lean`, `normalizeInstance`
to `wrapInstance`, and the trace class `Meta.instanceNormalForm` to `Meta.wrapInstance`,
removing the "instance normal form" terminology from both documentation and code.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>