perf: inline `EnvExtension.getStateImpl`

`getStateImpl` needs an `Inhabited` instance only to panic on an invalid extension index, but as an out-of-line call it made its callers build the instance first. `PersistentEnvExtension.getModuleEntries`, used by every lookup in an imported module's entries, thus allocated and freed an `Inhabited (PersistentEnvExtensionState α σ)` on each call. Inlined, the instance is only built on the panic path. No public IR signature changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>