nostr: separate next-branch advert namespace by default

Bump the default Nostr-discovery advert namespace from
`fips-overlay-v1` to `fips-overlay-v1-next` on next. Master
continues to publish under `fips-overlay-v1`.

Background: next runs FMP-v1 (Noise XX, msg1 33 bytes) which is
wire-incompatible with master's FMP-v0. Until now both branches
defaulted to the same Nostr advert namespace, so a stock
next-branch daemon's open-discovery sweep would happily pick up
master peers' adverts (and vice versa), succeed at the UDP punch,
adopt the socket, and fail every FMP handshake at the version-gate.
The per-peer-mismatch cooldown introduced on master is the safety
net for any case that slips past this default; the namespace
separation is the structural answer.

Three sites updated:

- `src/discovery/nostr/types.rs` `ADVERT_IDENTIFIER` const
  documents why the value is branch-specific.
- `src/config/node.rs` `default_app()` matches.
- `src/discovery/nostr/tests.rs` and the
  `testing/nat/scripts/nostr-relay-test.sh` malformed-advert
  fixture publish under the new namespace so test harnesses see
  the same adverts a real daemon would.

Operators who need cross-branch discovery during a coordinated
rolling upgrade can override `node.discovery.nostr.app` in
fips.yaml back to `fips-overlay-v1`.

testing/nat/scripts/nostr-relay-test.sh

@@ -191,7 +191,7 @@ pubkey = xonly_pubkey(secret).hex()
 created_at = int(time.time())
 kind = 37195
 tags = [
-    ["d", "fips-overlay-v1"],
+    ["d", "fips-overlay-v1-next"],
     ["app", "fips.nat.lab.v1"],
 ]
 content = "this-is-not-a-valid-overlay-advert-{garbage}"