subprocess: capture_output, timeout, stderr=PIPE/STDOUT, communicate, check

Found 2026-10-19 in sysdash's backend.py, which calls subprocess.run([...], capture_output=True, text=True, timeout=5). lib/rtl/subprocess.pas refused capture_output, timeouts, stderr=PIPE/STDOUT and .communicate() on purpose, and stderr=DEVNULL was accepted but not honoured. A collector streaming over ssh needs the timeout most: a hung nvidia-smi stalls the stream.

Fix:

• PAL: PalVforkAndExec3 also wires the child's stderr, with -2 meaning stderr=STDOUT. POSIX only; ESP and wasi return unsupported. The old subprocess.pas header named this work feature-pal-spawn-stderr-redirect, but that ticket was never filed.
• run() returns a CompletedProcess (args/returncode/stdout/stderr, check_returncode). It takes capture_output=, stdout=, stderr=, stdin=, text=, universal_newlines=, timeout= and check=. On timeout it kills the child and raises TimeoutExpired, as CPython does.
• Popen.communicate(timeout=) added, and Popen.stderr and .args. Both pipes are drained as one poll set, so the child cannot deadlock.
• Text mode translates line endings to \n, as CPython's universal newlines do.
• SubprocessError, TimeoutExpired and CalledProcessError use CPython's hierarchy and messages.
• A missing program now raises FileNotFoundError in the parent, and a non-executable one PermissionError. Before, the child exited 127, so except OSError never ran.
• An unsupported redirect raises ValueError instead of Halt(1).

Two traps hit while building it:

1. Inside Popen.Create, the bare constant STDOUT meant the stdout PARAMETER, because Pascal is case-insensitive, so stderr=STDOUT silently merged nothing. The code now uses the alias STDOUT_REDIRECT.
2. Exception classes must be declared in the unit the program imports. A class in a unit that unit merely uses is not found by except subprocess.TimeoutExpired:. pylib comes LAST in the uses clause so that Exception is pylib's (mimic_sqlite3.pas, note 1).

Not here: shell=True, env=, input=, wait(timeout=).

Test: test/test_nilpy_subprocess_capture.npy. Both existing subprocess tests still pass.