Fix Elixir 1.20 type-checker warnings

[simoncocking]

Summary

Elixir 1.20's new type checker surfaces two warnings in consuming applications. Both are fixed here with no behaviour change.

1. Redundant clause in generated struct channel/2

The use Amplified.PubSub macro generated:

def channel(%module{id: id}, ns) do ... end
def channel(_subject, _ns), do: nil

Each impl is for: a single struct, so the checker sees %module{id: id} as covering every value of that struct and flags the nil fallback as redundant — one warning per consuming module (100+ in parasol).

Fixed by collapsing to a single clause that checks for :id via Map.has_key?/2. A plain function call is opaque to the checker, so the nil branch stays reachable. Behaviour is identical: structs with an :id (including nil, which derives "thing:") get a channel; structs without one return nil.

Note: an is_map_key/2 guard does not work — the checker narrows guards too and re-flags the fallback. It must be a plain call.

2. Deprecated default argument in the protocol

Amplified.PubSub.Protocol declared def channel(subject, namespace \\ nil). Default args in defprotocol are deprecated in 1.20 ("protocols can only define functions without implementation"). Split into explicit channel/1 and channel/2; the Tuple impl gains a channel/1 head.

Testing

• mix compile --force --all-warnings is clean.
• All 66 tests pass.
• Verified against parasol: build goes from 9 of these warnings to 0.