Log window: stream the hook's logs over IPC into an in-app Log panel

Add a shared log ring (common/coop/log_ring.hpp): a lossy multi-producer /
single-consumer ring named coop_log_<pid>. The hook logs from several threads,
so producers claim a slot with fetch_add and publish each record with a
release store of its sequence; the consumer reads in order and tolerates
losing the oldest lines if it falls a whole ring behind.

The DLL's logf() now formats once and pushes every line to the ring (the file
trace stays as an opt-in mirror); the worker attaches the ring right after IPC
connect so bring-up is captured. The host (IpcServer) creates the ring at
injection time and exposes drain_logs(); a new LogPanel pulls new lines each
frame into a bounded rolling buffer and renders them with auto-scroll, a
filter, and clear. Added to the View menu (and UiState.show_log).

Verified against Phantom Brave via coop_audio_probe, which now also creates the
ring and drains it: the full hook bring-up trace streamed over IPC. All four
tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -125,6 +125,11 @@ Done:
   control is disabled when input forwarding is off, and the Audio panel notes when
   the render-hook is off (mirroring then uses loopback). Defaults to all-on so
   behavior is unchanged unless you toggle something.
+- **In-app Log window. ✅** The injected DLL streams its log lines to the host
+  over a shared log ring (`coop_log_<pid>`, a lossy multi-producer ring), and the
+  host shows them in a **Log** window with auto-scroll, a text filter, and clear.
+  Replaces tailing `%TEMP%\coop_hook.log` (which stays as an opt-in file mirror).
+  `coop_audio_probe` drains and prints the same stream headless.
 
 Future work, roughly in priority order: