Add CLIENT_CORE.md, and run event-model and client-core in run-tests.sh

CLIENT_CORE.md is the map for the crate: what holds what against the
Kotlin it replaces, how many tests were ported per file (85 total,
test-for-test where the Kotlin had JVM tests), what api.rs and
transcript_fold.rs cover versus don't yet, and the two things left
deliberately undone with reasons (TranscriptUnits.kt's Compose-specific
flatten, and event_model::Event's missing Unknown catch-all).

run-tests.sh now loops event-model, client-core and server rather than
only server, so the new crates' tests run from the same one command
AGENTS.md already points at.

Note for whoever merges this into rustify: this worktree branched
before RUST.md existed there, so I could not apply the requested edit
to its "Where things stand" bullet without an add/add conflict against
concurrent work on that file. Suggested wording is in this commit's
message on the orchestrator side -- apply directly to rustify's
RUST.md: mark item 1 of the Recommendation and the "Not started:
client-core" bullet as done, pointing at client-core/ and
CLIENT_CORE.md, dated 2026-09-04.

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

run-tests.sh

@@ -1,11 +1,18 @@
 #!/bin/sh
-# Runs this repo's tests. Extra arguments are forwarded to `cargo test`,
-# e.g. `./run-tests.sh transcript` to run just the transcript tests.
+# Runs this repo's tests. Extra arguments are forwarded to each `cargo test`,
+# e.g. `./run-tests.sh transcript` to run just the transcript tests in all
+# three crates.
 #
-# Only `server/` has tests: it holds all the logic worth testing (event
-# normalization, transcript cursors, config persistence, token auth), while
-# the Android app is UI over its HTTP API. Verifying the app means running
-# it -- see AGENTS.md.
+# server/ holds the backend's own logic (event normalization, transcript
+# cursors, config persistence, token auth); event-model is the wire shape
+# server/ and client-core share; client-core is the app's pure logic held
+# once instead of twice (see CLIENT_CORE.md) -- the syntax highlighter, the
+# ANSI parser, the transcript cache and fold, the REST and SSE clients. It
+# is not wired into the Android app yet (RUST.md), which still carries its
+# own Kotlin copies and has no tests of its own to run here; verifying it
+# means running it -- see AGENTS.md.
 set -eu
-cd "$(dirname "$0")/server"
-exec cargo test "$@"
+cd "$(dirname "$0")"
+for crate in event-model client-core server; do
+  (cd "$crate" && cargo test "$@")
+done