feat(companion): fipssh — ssh to a mesh node by npub (Termux helper)

Verified against the fips crate source: the mesh ULA is a pure function
of the PUBLIC key — fd || sha256(x-only pubkey)[0..15] — so the npub is
the durable address and needs no resolver. Android/tools/fipssh wraps
ssh for Termux: 'fipssh user@npub1…' derives the ULA (pure-python
bech32 + sha256, checksum-validated, typo protection) and execs ssh
over the companion's split tunnel; --resolve prints the ULA alone.

The derivation is pinned by a new Rust test
(npub_derives_the_same_mesh_ula_as_the_fips_identity, 3 seeds against
fips::Identity) and the helper's output was verified byte-identical
against a live fips identity pair. SSH-over-mesh handover updated with
an addendum: node docs/UI can advertise npub-based addressing, no
node-side DNS needed for this case.

Android/rust/archy-fips-core/src/nostr.rs

@@ -795,4 +795,30 @@ mod tests {
         );
         assert!(secret_from_nsec("npub1").is_err());
     }
+
+    /// The mesh ULA is a PURE function of the node's public key:
+    /// `fd ‖ sha256(x-only pubkey)[0..15]` (fips identity/node_addr.rs →
+    /// identity/address.rs). That is what makes "address by npub" work —
+    /// Termux's fipssh helper, and any future DNS-style resolver, just
+    /// computes what the fips daemon's DNS answers.
+    #[test]
+    fn npub_derives_the_same_mesh_ula_as_the_fips_identity() {
+        for seed in [0x42u8, 0x07, 0x31] {
+            // 0xff… would exceed the curve order — secret keys must be valid scalars.
+            let secret = [seed; 32];
+            let id = fips::Identity::from_secret_bytes(&secret).unwrap();
+            let npub = id.npub();
+            let expected = id.address().to_ipv6().to_string();
+
+            let pubkey_hex = pubkey_from_any(&npub).unwrap();
+            let pk = hex::decode(&pubkey_hex).unwrap();
+            let mut hasher = Sha256::new();
+            hasher.update(&pk);
+            let hash = hasher.finalize();
+            let mut ula = [0u8; 16];
+            ula[0] = 0xfd;
+            ula[1..].copy_from_slice(&hash[..15]);
+            assert_eq!(std::net::Ipv6Addr::from(ula).to_string(), expected, "npub {npub}");
+        }
+    }
 }