[codex] Add robust exec-chain profiling support

[wangyinz]

Summary

• Add robust PEAK exec-chain support for exec*, fexecve, execveat, posix_spawn, and posix_spawnp paths.
• Add non-destructive rank-local exec checkpoint output and CSV tracing controls without calling peak_fini() before exec.
• Preserve profiling across custom envp by injecting/de-duplicating LD_PRELOAD and propagating missing PEAK_* variables when enabled.
• Interpose dynamically linked syscall(SYS_execve/SYS_execveat) so raw syscall callers do not silently bypass the exec-chain path.
• Add tests and README documentation for PEAK_EXEC_CHAIN, PEAK_EXEC_CHECKPOINT, PEAK_EXEC_PROPAGATE_PEAK_ENV, and PEAK_EXEC_TRACE_PATH.

Root Causes Addressed

• Successful exec replaces the old image before PEAK finalization, losing in-memory profile data unless a non-destructive checkpoint is emitted first.
• Custom child environments can omit LD_PRELOAD and PEAK_*, preventing the replacement image from being profiled.
• Vista/NVHPC AArch64 exposed an inline raw-syscall miscompile in exec/checkpoint paths; both paths now share a safer raw syscall bridge.
• Frontera exposed a timer ABI issue from resolving the compat timer_create symbol; the signal policy now resolves the GLIBC_2.3.3 interface.

Validation

• Local full CTest: 346/346 passed.
• Local static check: test_cuda_interceptor_consistency passed; git diff --check clean.
• Vista smoke via hpc-test: job 791735, COMPLETED 0:0, exec-chain smoke passed.
• Frontera smoke via hpc-test: job 7832267, COMPLETED 0:0.
• Frontera MPI slice via hpc-test: exec checkpoint tests passed; one unrelated strict-detach MPI test failed, consistent with pre-existing broad-suite noise.
• Frontera main baseline via hpc-test: job 7832231 failed 47/216, used as evidence that remaining broad-suite failures are not introduced by this branch.

Review

Three independent final review rounds completed with no blockers found.