python: concurrency and the sync contract

Created with Claude Code from the spec in #1341.

Parent

Sub-issue of #1341. Ticket 13 of 14.

The concurrency and interrupt contract from #1341 holds from the public API:

Two threads entering their first operation at the same time build one engine client and load the default keyset once.
Two Cipher instances for different models share that client.
Concurrent encrypt and decrypt from several threads all succeed and all rows open.
A KeyboardInterrupt during a long encrypt behaves as ticket 3 found: it lands after the detached call returns, and the Cipher is usable afterwards. If ticket 3 amended the fact, implement and test the amended behaviour.

A test with a barrier releases 8 threads into their first encrypt; the client-built counter reads 1 and the keyset-load counter reads 1.
8 threads × 100 encrypts of distinct values, then one decrypt of all 800, all equal.
A subprocess test sends SIGINT during a 5000-value encrypt; the process observes KeyboardInterrupt exactly once, the call did not leave a half-built result, and a subsequent encrypt succeeds.
The README documents the sync-only, GIL-released, one-client-per-process contract in a short section.