[bahdotsh]

Step 6 of the shared network plan (ADR 0037). First of a stack: steps 7 to 12 are stacked on this branch.

What an application gets

• ProtocolConfig.app_publisher_key: optional, 32 bytes. Any other length is refused at construction, so a key that was meant to be set never silently configures nothing.
• set_app_membership(cert) and app_membership_status() on the engine. The status reports this node's own configuration only (configured, its certificate's not_after while current, how many neighbours are visible).
• sign_app_membership(publisher_private_key, address, not_after): a pure namespace function for embedded mode (decision 6a). A source guard (sign_app_membership_never_lets_the_key_out) refuses any log, event, storage or send in its body. I checked it with a negative control: planting a debug! in the body makes it fail.

Nothing is gated yet. Visibility is still read only by tests; the gate is the next PR.

#685 review items folded in

1. No restart when the same certificate is supplied again. Before this, an app that hands the certificate over on every foreground made every neighbour sign and seal again. A hello burst with no link change is also something only a configured node produces. A renewed certificate still restarts every exchange (tested).
2. No hello for a peer with no mesh link. Discovery also reports relay, Nostr and LAN peers. Each of those cost a Lamport tick and an Ed25519 signature before send_to_neighbor refused it. A later mesh link is still greeted at once (tested).
3. Tests for the 512-exchange and 1024-visible bounds.

Regenerated

The whole set was regenerated from the one definition: Swift, Kotlin, Python, Ruby, Java, the C dispatcher, the C header (no change), the Python local API table, and the C# generated sources.

• C# generator: it refused sequence<u8>? on ProtocolConfig, which is what it should do. It now has a rule for that type: standard base64, the C API's JSON form for bytes.
• Local API: the three calls are platform operations.
  • The certificate binds the one identity every client shares, so supplying it is the operator's job.
  • The signing helper takes a private key, which must never cross a socket.
• Java: ConsumerCheckTest.config() passes null for the new field.

Tested locally

• cargo test -p offline-protocol --lib membership: 29 pass, plus the two signing tests.
• cargo test -p offline-protocol-uniffi: 318 pass, including the two new FFI tests.
• Generator checks:
  • capi-dispatch-gen --check: clean.
  • generate_local_api_table.py --check: clean.
  • gen_csharp_local_api.py --check: clean.
• Python tests/local_api: 129 pass.
• Clippy: cargo clippy --workspace and --no-default-features on offline-protocol are both clean.
• Rustdoc under -D warnings: clean.

Not run locally

Java, Ruby, C#, Swift and Android. CI runs those.

Owed to later PRs in the stack

• React Native and Flutter wrapper surfaces for the three calls.
• The integrator guide (step 12).