Reject negative descriptors in single-descriptor readiness helpers

The issue is in std/net/poll.wave, specifically the functions net_wait_readable, net_wait_writable, and net_wait_connected. Start by examining the Linux poll handling in that file. Add validation to reject negative file descriptors (return -9). Ensure infinite timeout does not cause indefinite wait. Write a regression test using the provided reproduction script, and verify that normal behavior and disabled entries in net_poll are preserved.

Problem

On Linux, net_wait_readable, net_wait_writable, and net_wait_connected in std/net/poll.wave return a timeout result for a negative descriptor instead of a bad-descriptor error. Linux poll ignores negative entries. Source inspection therefore also indicates an indefinite-wait risk with an infinite timeout; the executed reproduction below uses a zero timeout.

Reproduction

Save as repro.wave:

import("std::net::poll")::{net_wait_readable};
fun main() -> i32 {
    if (net_wait_readable(-1, 0) == -9) { return 0; }
    return 1;
}

From the repository root, stage the matching std and run:

audit_home="$(mktemp -d)"
mkdir -p "$audit_home/.wave/lib/wave"
cp -R std "$audit_home/.wave/lib/wave/std"
HOME="$audit_home" target/release/wavec build repro.wave -O0 --run
HOME="$audit_home" target/release/wavec build repro.wave -O2 --run

Tested at 01955d715dda275de993948037091140bb859174 on Fedora Linux 43, x86_64, with LLVM 21.1.8, at both -O0 and -O2.

Expected: the helper returns -9 and the program exits 0. Actual: it returns 0 and the program exits 1, with no output. The other two helpers behave the same way. An invalid positive descriptor (2147483647) correctly returns -9 in all three helpers.

Scope

Validate negative POSIX descriptors at the single-descriptor helper boundary. Preserve the disabled-entry behavior of generic net_poll on POSIX targets. Windows uses Winsock socket handles rather than POSIX descriptors; do not infer its validation or error mapping from this Linux reproduction. Platform error classification in #508 is separate.

Completion criteria

• All three helpers return -9 immediately for negative descriptors on Linux, including with an infinite timeout; bound the regression process with an external timeout.
• Add regressions for negative and invalid positive descriptors.
• Preserve normal readable, writable, and timeout behavior, and disabled entries in POSIX net_poll.

Difficulty: easy.