Preserve interpreter-boundary diagnostics and syslog restrictions - #9365

Summary

Preserve exception diagnostics across two existing production VM boundaries:

• Build _thread unraisable-hook messages from the callable's WTF-8 representation instead of replacing lone surrogates through Rust Display.
• Wrap code/function cross-interpreter marshal failures as NotShareableError, preserving the original exception as __cause__ and CPython's serialization/deserialization message.

The existing Option<String> unraisable entry point remains unchanged. Hook/audit dispatch, failure handling and finalization behavior are preserved. This does not change the lossy stderr fallback or other generator/deallocator message producers.

The marshal change is limited to two existing private helpers. Successful operations and existing callers are unchanged. No new cross-interpreter mode, test C API, subinterpreter lifecycle, GC ownership, native-monitoring API or fixture registration is included.

CPython 3.15 sources: thread producer, Unicode formatting, marshal error boundaries, code/function callers.

Validation

All four gates passed on exact composed head 3df6951343b5; normal commit hooks also passed.

Four compile-only gates cover VM-library cargo check and strict cargo clippy -- -D warnings, using --no-default-features --features compiler,host_env,stdio,gc,doc with and without threading. All use locked/offline dependencies and Rust 1.99.0. These are selected feature sets, not Cargo default features or full-workspace validation.

No runtime tests were run. The existing test_thread.ThreadRunningTests.test_unraisable_exception starts a thread and covers an ordinary representation, not the surrogate boundary. Canonical XI tests require native fixtures or interpreter lifecycle work outside this patch. No test markers or fixtures are changed; full workspace and C API tests remain unrun.

RustPython's existing marshal path calls the Python module rather than CPython's direct C implementation; this pre-existing difference remains. This patch is not a claim of full serialization parity.

AI assistance: OpenAI Codex (model identifier unavailable), also disclosed in the commit trailers.

Queue and syslog interpreter-boundary follow-up

Match the CPython 3.15 syslog guards: reject subinterpreter openlog() and closelog() before auditing or host mutation, and reject implicit syslog() opening after its audit event when the logger has not already been opened by the main interpreter. Already-open logging and setlogmask() are unchanged. This does not redesign global logger locking or interpreter lifecycle.

Migrate Lib/concurrent/interpreters/_queues.py through scripts/update_lib to that exact CPython commit, removing the unused eager pickle import and exception targets. Native serialization imports pickle on demand. Public import timing, incidental module attributes, and debug exception locals can nevertheless differ; this is not claimed to be unobservable.

The follow-up source tree was recovered exactly and validated anew in a fresh workspace using official Rust 1.99.0 and normal repository hooks. Four sequential, bounded, locked/offline compile-only gates cover default CLI cargo check, frozen-stdlib CLI cargo check, cargo check -p rustpython-stdlib --lib, and strict cargo clippy -p rustpython-stdlib --lib -- -D warnings. Linux default stdlib features compile the Unix syslog guards. Full source, toolchain, and extracted dependency-source hashes/membership were checked before and after each gate. System Python/native OS libraries remain an explicitly trusted managed-environment boundary, not a fully hermetic build claim.

No Python module imports, syslog calls, queue/thread/interpreter-lifecycle, or serialization runtime tests were run for this follow-up. No fixtures or test selections were changed. Full workspace and C-API runtime suites remain unrun. Earlier VM-feature receipts above describe their original head, not a rerun on this follow-up tree.

Exact follow-up head: 5fb5e68cdc31868e1c43c6787dc27ba334410221 (tree 7d406e46c4cc9c7592e3aa5eeccdaf59f3f2ba47). All four fresh compile-only gates and normal commit hooks passed. Prior public head 3df69513 had all 25 hosted CI checks succeed in run 40191688907; that result does not validate this follow-up head. New-head hosted CI remains pending after publication.