client-core: the app's own log ring, and POST /client-log to get it off a phone

Iris tests iris builds on a phone with no adb, and Android forbids one
app reading another's logcat, so a `log::info!` in the app can only reach
her if the app carries its own copy and sends it somewhere.

`client_core::log_ring` is that copy: a bounded ring (2000 lines / 256
KiB, whichever bites first) behind a `log::Log` backend that forwards to
whichever real logger the platform installed, so `logcat` and the desktop
terminal see exactly what they saw before. Reading does not consume --
the report and the uploader are two readers of one ring.

`client_core::log_upload` drains it into ai-server's new `POST
/client-log`, which re-emits each line into the server's own tracing
output. Dev Updater already shows that as ai-server's runtime log, so
nothing new is built there. A failed batch is retried from the same
cursor, and nothing in the upload path calls `log!` -- it would land in
the ring it is draining.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client-core/Cargo.lock

@@ -47,6 +47,7 @@ name = "client-core"
 version = "0.1.0"
 dependencies = [
  "event-model",
+ "log",
  "pulldown-cmark",
  "serde",
  "serde_json",