Auth rules: behaviour unclear if no power level event is cited as an auth event

**Link to problem area**: https://spec.matrix.org/v1.2/rooms/v9/#authorization-rules and also https://spec.matrix.org/v1.2/client-server-api/#mroompower_levels

**Issue**
The auth rules say:

> **INFO:** Power levels are inferred from defaults when not explicitly supplied. For example, mentions of the sender’s power level can also refer to the default power level for users in the room.

I think this is talking about the situation where the auth rules need to lookup the power level of `@alice:alice.com` from some state map, but Alice does not have an explicit power level.

What happens if the event *E* under consideration does _not_ cite an auth event of type `m.room.power_levels`?

Options that spring to mind:
1. Reject *E* outright (making an exception for [the initial bit of room state added after the room is created](https://spec.matrix.org/v1.2/client-server-api/#post_matrixclientv3createroom)).
2. Look for a power levels event in the auth chain of *E*, rather than just *E* itself. (But perhaps this need not exist---what then?)
3. Use the default power levels [given in the C-S spec](https://spec.matrix.org/v1.2/client-server-api/#mroompower_levels) as the intended power levels when there is no such power level event in the room.
* If this is the case, the wording in the C-S spec is unhelpful. It reads
> if the room contains no `m.room.power_levels` event [...]

but it's possible that the room contains an `m.room.power_levels` event somewhere else in the DAG---it's just that one hasn't been cited as an auth_event here.

The wording of the **INFO** box in the auth rules makes me think option 3 is intended. Can anyone confirm or refute?

Start with the authorization-rules and m.room.power_levels sections of the Matrix v1.2 specification linked in the issue, then review the seven comments for an agreed interpretation. Done means resolving whether a missing cited power-level event has defined behavior and recording that behavior clearly in the relevant specification text.

Written by the indexing model from the issue text.