test: serialize the frontend acceptance stand-in's shared state

[roodboi]

Summary

Fixes a lost-update race in the native frontend acceptance controls' stand-in runtime (tests/python/test_native_frontend_acceptance.py, added by #99). It makes the Runtime state models job fail intermittently, as on #112 at ea0e0d07.

• Cause: stand-in invocations run concurrently (a foreground up while the driver polls ps), and each did an unlocked read-modify-write of a shared state.json. A ps that loaded the state before the second up saved its foreground token then saved its stale copy over it. up saw its token gone and exited 0 before readiness ("up-second exited 0 before readiness"); lost call records also failed the positive acceptance test.
• Fix: each invocation holds an exclusive flock across its read-modify-write and releases it only before its long waits (the foreground loop and the ps-hang fault). No assertion changes.

This is the same patch as commit 70c78f77 on #112, cherry-picked onto next so it can land independently of the socket work.

Verification

• Unfixed: 26 of 40 local runs of the controls failed, 8 with the exact CI message; the rest were lost call records.
• Fixed: 0 of 40 failed.
• python3 -m unittest tests/python/test_native_frontend_acceptance.py passes on this branch; privacy ok.

Refs HACK-1211.

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[linear-code]

HACK-1211

[roodboi]

This exact frontend acceptance stand-in patch is included in merged PR #112 (f00a6478). The file blob at this branch and #112 is identical (09329f82). Closing this duplicate PR as superseded; the qualified branch and all discussion remain preserved.