Harden subprocess wait and signaling primitives

[floitsch]

Summary

• make repeated native subprocess wait registration idempotent
• prevent intrusive-list corruption when a wait is interrupted and retried
• propagate POSIX kill failures instead of silently discarding them
• propagate Windows TerminateProcess failures and only mark a process killed after success
• add direct regression coverage for repeated registration and signaling errors

The cancellation-safe pkg-host waiter is toitlang/pkg-host#98. The supported Process termination API is toitlang/pkg-host#99.

Tests

• cmake --build build/host --target toit -j4
• build/host/sdk/bin/toit analyze tests/subprocess-wait-registration-test.toit
• build/host/sdk/bin/toit run tests/subprocess-wait-registration-test.toit
• rebuilt SDK passed the pkg-host process kill regression