Fix Tor onion adverts missing port in Nostr overlay discovery

The Nostr overlay advert publisher serialized `transport: tor`
endpoints as a bare `<onion>.onion` hostname with no port. The Tor
address parser requires `<host>:<port>` form and rejected the bare
shape with `expected host:port`. Any peer receiving a Tor-only
advert went into a persistent retry-fail loop on jittered backoff
until the advert aged out of the discovery cache. The bug had been
latent for as long as Tor adverts have been published on Nostr, and
was masked in deployments where every node also advertised a
non-Tor transport (peers fell through to the working endpoint).
Surfaced first on a deployment where Tor was the only advert path.

Publisher now emits `<onion>.onion:<port>` using a new
`transports.tor.advertised_port` config field that defaults to 443,
matching the Tor `HiddenServicePort 443 127.0.0.1:<bind_port>`
convention. Operators whose torrc uses a non-default virtual port
can override.

Adds a unit test that pins the publisher/parser contract: formats
the advert exactly as the publisher does and asserts `parse_tor_addr`
accepts the result; asserts the bare-onion form (the bug) does not
parse, catching any future regression that drops the port again.

Parser is unchanged (already correct).

docs/design/fips-configuration.md

@@ -467,6 +467,7 @@ Requires an external Tor daemon providing a SOCKS5 proxy. Three modes:
 | `transports.tor.max_inbound_connections` | usize | `64` | Maximum inbound connections via onion service. |
 | `transports.tor.directory_service.hostname_file` | string | `"/var/lib/tor/fips_onion_service/hostname"` | Path to Tor-managed hostname file containing the `.onion` address. |
 | `transports.tor.directory_service.bind_addr` | string | `"127.0.0.1:8443"` | Local bind address for the listener that Tor forwards inbound connections to. Must match `HiddenServicePort` target in `torrc`. |
+| `transports.tor.advertised_port` | u16 | `443` | Public-facing onion port published in Nostr overlay adverts. Must match the virtual port in torrc's `HiddenServicePort <port> 127.0.0.1:<bind_port>` directive — that is the port other peers will use to reach this onion. |
 
 **Named instances.** Like other transports, multiple Tor instances can
 be configured with named sub-keys for different SOCKS5 proxy endpoints.