Fix red CI on main, a stale standing context, and three Neural Engine helper bugs

[huyedits]

Why CI on main is red

83e6534 (side models on the ANE) rewrote tests/conftest.py from a copy older than the one on main. That removed the hook that reports MLX-only tests as skips off a Mac. On Linux CI that became 126 failures + 9 errors, all Symbio's inference engine is not installed. On a Mac the hook does nothing, so nothing showed up locally.

• The hook is restored next to the new no_neural_engine_helper fixture.
• tests/test_conftest_hooks.py now fails on any host if it disappears again.

Bugs found reviewing today's merges

Standing context never refreshed in long sessions (symbio/app/chat_turn.py). Curated memory and the env note are snapshotted and meant to be re-read every STANDING_REFRESH_TURNS turns. The turn count came from the number of user messages in self.history. But _trim_history pops from the front once the history is at its cap, so that count stops rising, and a long session never saw a memory or soul change again. Turns are now counted directly.

Neural Engine helper (symbio/app/ane.py, symbio/app/decider.py):

• ane.request() says it never raises, but a helper binary that failed to exec raised OSError out of _start() into see_screen and routing. It now returns ok: False.
• A decide() that timed out killed the helper without recording a back-off. Every turn then paid the 3 s wait, a helper restart, and the next OCR's ~7 s cold load. A timeout now backs off the same way "unavailable" does.
• decider.decide() re-ran a vote that had already failed after Apple's model also failed, doubling the wait when the helper is down.

Testing

• Every fix has a test that fails without it and passes with it. The standing-context test stubs the sampler and mlx.nn, so it runs off a Mac too.
• Full suite in a fresh virtualenv on Linux: 2477 passed, 250 skipped, 0 failed.