feat: introduce client event system (#106)

* chore(flake): accept extra system attr; add perl for openssl-sys build

forAllSystems calls the lambda with {system, pkgs}; strict
destructuring requires `..` to ignore the system attribute.

`pkgs.perl` is needed because openssl-sys is pulled vendored via
libsqlite3-sys / rusqlite / chat-sqlite, and its `perl Configure`
step needs FindBin.pm, which Fedora's system perl doesn't ship.

* feat: introduce client event system

- Core processing yields a `PayloadOutcome` enum — `Empty`, `Convo`, or
  `Inbox`. `ConvoOutcome` carries a conversation id and an optional
  decrypted `Content`; `InboxOutcome` adds a `NewConversation`
  (id + `ConversationClass`) for a peer-initiated conversation.
- Client translates `PayloadOutcome` into app-facing `Vec<Event>`
  (`ConversationStarted`, `MessageReceived`) at the boundary, so the
  application loop sees discrete events rather than core types.
- MLS group welcomes produce a `ConversationStarted` event with no
  initial content, fixing the silent-group-join case where the inbox
  layer dropped the observation.
- C FFI exposes an `EventList` opaque type with indexed accessors and
  an `Invalid` sentinel for out-of-bounds / non-applicable reads.
- Symmetric `Inbox` / `InboxV2` handlers: both return
  `Result<InboxOutcome, _>` and own the persistence + ephemeral-key
  cleanup for the conversations they create.
- Updated and simplified `docs/adr/0001-client-event-system.md`.

* chore(flake): bump nixpkgs to nixos-unstable-small

Temporary. The two crates.io UA fixes (NixOS/nixpkgs#512735 for
fetchCargoVendor's python-requests UA, NixOS/nixpkgs#524985 for
importCargoLock's curl UA) haven't propagated to nixos-unstable yet.
Switch to nixos-unstable-small and force logos-delivery to follow so
the smoketest gets the same fix. Revert once nixos-unstable catches up.

Refs:
- https://github.com/rust-lang/crates.io/issues/13482
- https://github.com/rust-lang/crates.io/issues/13783
- https://crates.io/data-access

.github/workflows/ci.yml

@@ -73,10 +73,15 @@ jobs:
         working-directory: crates/client-ffi/examples/message-exchange
       - name: Build logos-delivery
         run: nix build .#logos-delivery
+      # Build and run chat-cli through the dev shell so it links against the
+      # same Nix glibc as the prebuilt liblogosdelivery.so. A plain `cargo
+      # build` uses the runner's system glibc, which is older than Nix's and
+      # mismatches it at runtime (libc.so.6: version `GLIBC_ABI_DT_X86_64_PLT'
+      # not found, required by Nix glibc's libm.so.6).
       - name: Build chat-cli (logos-delivery)
-        run: LOGOS_DELIVERY_LIB_DIR=./result/lib cargo build --release -p chat-cli
+        run: nix develop -c bash -c 'LOGOS_DELIVERY_LIB_DIR=./result/lib cargo build --release -p chat-cli'
       - name: Run chat-cli smoketest
-        run: ./target/release/chat-cli --name ci-test --smoketest
+        run: nix develop -c ./target/release/chat-cli --name ci-test --smoketest
 
   nix-build:
     name: Nix Build