GeezerVison

Summary

Create the C API worker VM through Interpreter::new_thread() instead of temporarily entering the template VM while holding MAIN_INTERP. The existing attachment path still attaches the returned worker after the factory returns and releases that mutex.

On current main, both calls use the same interpreter receiver and clone implementation, preserving the worker's owned shared state. The old path can park during attachment while retaining MAIN_INTERP, including shutdown parking that can prevent later registration cleanup from acquiring the mutex. This is a source-supported possible interleaving, not a runtime-reproduced shutdown race.

This change removes that attachment-under-mutex edge. It does not remove the existing trace/profile locks or establish general callback, admission, retirement, or finalization safety.

Regression test and validation

Add one native structural test using a fresh OS thread. Merely constructing a worker must leave both the VM stack and current thread slot empty. The parent detaches while joining, then drops the returned clone under its attached interpreter before assertions. No shutdown, stop-the-world, race, or callback probe is performed.

On Linux x86_64:

• The identical test on the original factory failed exactly the final cached-slot assertion: (false, true) instead of (false, false).
• With this change, the exact test passed: 1 passed, 0 failed, 0 ignored, with 130 other tests filtered out.
• Built the lib-test using cargo test --locked --offline -j1 -p rustpython-capi --lib --no-run, from crates/capi, and executed only pylifecycle::tests::requesting_thread_vm_does_not_attach from separately preserved baseline/candidate executables. Neither links host libpython.
• cargo clippy --locked --offline -j1 -p rustpython-capi --lib --tests -- -D warnings passed, also from crates/capi.
• Normal commit hooks passed. The full C API suite was not run.

The tested base is 0231d847d9da0eed90c3fe87010205683d52af56. Main subsequently advanced to f65bf8e52d0c0a387bca897614befa1aa4af9d69 with a Cargo.lock-only dependency update. Source-only merge composition is clean and preserves this exact C API file; that newer-main combination was not executed locally.

Upgrade integration

This deliberately extracts a current-main prerequisite overlapping #8944, rather than claiming independent ownership. The same caller replacement also appears in the broader GC redesign #8939. #8944 already has the same caller replacement, but its Interpreter::new_thread() enters through enter_raw. This test is an explicit integration gate: that owner branch must preserve the factory's no-attachment boundary even when the textual caller change merges cleanly.

This PR does not use its restricted idle-anchor API, change its owner/admission design, or modify #8944.

AI assistance was used in source review, implementation, and validation.