fix(executor): do not retain callbacks when worker startup fails

[realAaronWu]

Purpose

Fixes #436. Related to #435, but does not implement planning cancellation or budgets.

DefaultExecutor::Add currently queues its callback before lazy worker startup. If startup throws, a later submission can run the callback whose submission failed.

Start workers under the existing mutex before enqueueing the callback. A failed invocation releases its captures immediately; partial worker startup can leave idle workers and does not restore the configured worker count on retry.

Tests

• Add a Linux 64-bit GNU/Clang static-library regression target with submitting-thread-only allocation faults. Cover startup, partial startup, immediate capture release, recovery, shutdown, and queue growth with an accepted prefix. CTest bounds the executable to 30 seconds.
• Actual unmodified-main production archive fails the startup regression with exit 1 through the native runner; the fixed production target passes.
• Local Linux x86_64 / GCC 15.3 / Debug CMake + CTest: new target 2 passed; common 1,721 passed + 2 expected skips; core 2,351 passed; scan 87 passed; scan-and-read 511 passed; read 302 passed.
• Scoped ASan/UBSan/LSan: the executor/status implementation and the new/existing executor test translation units, 14 cases passed. This is not whole-project sanitizer coverage.
• Pinned pre-commit hooks passed. Public CI remains pending for this validation draft. Local network tests and optional Rust formats were disabled; ARM, TSAN, and GPU execution are not claimed.

RelWithDebInfo new/common targets also passed. Its wider build hit an unchanged GCC 15 -O2 -Werror=maybe-uninitialized warning in remote_lookup_file_manager_test.cpp; the identical compile fails on main too. Debug wider targets above completed without suppressing that warning or changing unrelated code.

API and Format

No public API or storage-format change. Custom executors, general ShutdownNow allocation-failure safety, and complete pool rollback are outside this fix.

Documentation

No new user-facing feature. The implementation comment explains the submission boundary.

Generative AI tooling

Generated-by: OpenAI Codex (desktop; model/version not exposed).

[realAaronWu]

Local validation and isolated internal review are complete. The public workflows are awaiting maintainer approval; could you please approve their execution? This remains a validation draft until the applicable CI checks complete.