fix(agc): bound the async log's exit flush

On Windows the atexit handlers of a DLL run after the process has ended
its other threads, so the logger thread can be gone with lines still
queued and the unbounded Flush waits forever (guest_shader_link timed
out on 4 of the last 7 completed main builds since #6371). Run the bounded
FlushAtEnd there instead.

Co-Authored-By: Claude <[email protected]>

docs/dev/ARCHITECTURE.md

@@ -125,7 +125,7 @@ flowchart LR
 
 - **Queue.** One logger thread drains a queue that holds at most 16 MiB of waiting lines; the batch the logger is writing comes on top. A caller that finds the queue full blocks until the logger has taken the pending batch, so no line is dropped. The logger writes entries in queue order and joins consecutive entries of one stream; it writes them with one `fwrite` and flush when the text reaches 64 KiB, the stream changes or the batch ends. `AGC_LOG_DEFERRED` queues the format string with numeric, enum and `LogLiteral` arguments and formats them on the logger thread; `APS5_LOG_TIMESTAMPS` stamps a line when it is queued.
 - **Order.** Lines that pass through the sink, on stdout and stderr alike, appear in call order. A direct `fprintf(stderr, ...)` or `printf` does not pass through it and is written at once, so it can appear before older queued lines; the coalesced `deferred write-back failed` notes are of this kind. `APS5_LOG_SYNC=1` keeps one order for all of them.
-- **Flush.** `Flush` waits without a limit until every line queued before the call has been written, including while another thread holds the stream. It runs when the driver fails, in `Driver::stop` and at exit. `AsyncLog` also registers a bounded flush (`FlushFor`, 2 s) with `LibcSetEndFlush_nid_no_patch`; libc runs it where the process ends without those: `abort()` and `std::terminate` (a `SIGABRT` handler that the first registration chains in front of the previous one, so `raise(SIGABRT)` still ends the process), guest `abort()`, the Windows crash report of a fatal exception and guest `_Exit`. A title that installs its own `SIGABRT` disposition replaces that handler. The bounded flush polls two counters and takes no lock, so a thread that dies holding the queue lock or a stream cannot stall it. After 2 s it gives up, later calls return at once, and the lines still queued are lost.
+- **Flush.** `Flush` waits without a limit until every line queued before the call has been written, including while another thread holds the stream. It runs when the driver fails and in `Driver::stop`. At exit `AsyncLog` runs a bounded flush (`FlushFor`, 2 s) instead: on Windows the `atexit` handlers of a DLL run after the process has ended its other threads, so the logger thread may be gone with lines still queued, and a `Flush` there would wait forever. It also registers that bounded flush with `LibcSetEndFlush_nid_no_patch`; libc runs it where the process ends without `atexit`: `abort()` and `std::terminate` (a `SIGABRT` handler that the first registration chains in front of the previous one, so `raise(SIGABRT)` still ends the process), guest `abort()`, the Windows crash report of a fatal exception and guest `_Exit`. A title that installs its own `SIGABRT` disposition replaces that handler. The bounded flush polls two counters and takes no lock, so a thread that dies holding the queue lock or a stream cannot stall it. After 2 s it gives up, later calls return at once, and the lines still queued are lost.
 - **Statistics.** A `[log]` line every 10 s in which lines were logged reports lines, bytes, the time spent by the callers and by the logger, and the queue peak; `APS5_LOG_STATS=0` hides it. Identical `deferred write-back failed` notes print three times per 10 s and are then counted in one `repeated N more times` line.
 
 `agc_async_log` checks the order of deferred and written lines, long lines, the repeat limit, a `Flush` that waits for a held stream, a bounded flush that gives up, and a full queue that blocks its caller. `agc_async_log_fatal` starts the test executable as a child that queues lines behind a held stream and then calls `abort()`, calls `std::terminate()`, raises an access violation (Windows) or `SIGABRT` (elsewhere), or aborts without ever releasing the stream; the first four must leave every line in the file in order, the last must still end after the bound.