Extract the event model into its own crate, shared with client-core

RUST.md's recommendation item 1 starts here: Event, QuestionOption,
SessionStatus, ImageRef, AttachmentRef, SeqEvent, context_tokens and
context_after move to a new event-model crate so a future Rust client
shares one definition with server/ instead of Events.kt's hand-kept
mirror. session/driver.rs and session/transcript.rs re-export
everything they used to define, so nothing downstream of either
module changed.

cargo test (127 passed), clippy --all-targets and fmt clean in both
server/ and event-model/.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

server/Cargo.toml

@@ -13,6 +13,10 @@ path = "src/main.rs"
 # the RON house rules. Extracted from the two copies that had drifted --
 # see that repo's README for the evidence and the bug the extraction found.
 wg-app-link = { path = "../wg-app-link/server" }
+# The event model, shared with `client-core` so a Rust client and this
+# server read the same `Event`/`SeqEvent` rather than the app hand-mirroring
+# it the way `Events.kt` used to.
+event-model = { path = "../event-model" }
 axum = { version = "0.8", features = ["json", "multipart"] }
 axum-server = { version = "0.8", features = ["tls-rustls"] }
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "sync", "time", "process", "io-util", "signal"] }