Bounded-wait RPC companions: _within with timeout and cancellation

[turinglambdaai]

Closes #252.

What

The generated Windows/Linux C++ clients exposed only blocking calls, so hosts that poll or abandon an RPC spawned a detached std::thread per tick — threadly (MainWindow.Update.cpp:384) and movebit both churn one thread per 400 ms poll. Every RPC now also generates a bounded-wait companion:

std::optional<T> name_within(args, std::chrono::milliseconds rivet_timeout)
bool             name_within(args, std::chrono::milliseconds rivet_timeout)  // Void RPCs

The completion resolves a std::promise on the reader thread; the call waits up to the timeout and cancels the pending request through Backend::cancel on a miss — the existing request table does the bookkeeping, no new native thread. Backend and decoding errors still throw from the final get, matching the blocking contract. The naming collision check treats _within as a generated companion, so a schema RPC named foo_within fails generation instead of silently shadowing.

The checked-in Windows and Linux template headers gain the same shape in their own hand-written style, and docs/windows-native-window.md documents the sanctioned polling pattern: completions when a push model works; _within when the UI owns a tick loop — never a detached thread per tick.

Latent defect fixed

The new Void fixture exposed a latent generator defect: the Void async completion handler assigned into Result<void>, which has no value member — every generated header containing a Void RPC failed to compile (increment_within's fixture, log-line, now pins the corrected shape: the decode only validates arrival). Existing products with Void RPCs and generated _async calls were one regeneration away from this.

Tests

rivet/tests/codegen.rkt: the fixture gains a Void RPC (log-line), asserting increment_within/log_line_within shapes, cancel-on-miss returns, promise decode wiring, and the Void handler fix; existing snapshot-compatibility scenarios keep log-line present so their single-variable assertions stay single-variable. Full suite: 9868 pass.