Service manager step 1: Signal support (OP_PROCESS_SIGNAL)

Summary

Implement OP_PROCESS_SIGNAL for SIGKILL (kernel-level forced termination) and add a Signal::Terminate variant for message-based SIGTERM delivery via the HANDLE_PARENT channel.

This is step 1 of the service manager implementation (#26) and is also tracked separately as #24.

Details

• SIGKILL: The kernel immediately tears down the target process — reclaims memory, closes handles, unblocks waiters. No userspace code runs in the target.
• SIGTERM: Delivered as a Signal::Terminate message on the process's HANDLE_PARENT channel. The process can catch and handle it gracefully (e.g., flush buffers, close connections).

Key files

• panda-abi/src/lib.rs — add OP_PROCESS_SIGNAL opcode, Signal enum
• panda-kernel/src/syscall/ — syscall handler
• panda-kernel/src/process/ — forced termination logic
• libpanda/src/sys/mod.rs — low-level wrapper
• libpanda/src/process.rs — Process::kill(), Process::terminate()

Testing

• Kernel test: spawn, SIGKILL, verify termination and wait returns
• Kernel test: spawn, SIGTERM, verify message arrives on parent channel
• Userspace test: kill a looping child, verify clean exit
• Userspace test: kill a process with pending I/O, verify no deadlock

Documentation

• Doc comments on OP_PROCESS_SIGNAL opcode and Signal variants
• Update docs/IPC.md with signal delivery semantics

Part of #26

Start with the opcode and Signal enum in panda-abi/src/lib.rs, then read the syscall handler and process code under panda-kernel/. Trace the low-level wrapper in libpanda/src/sys/mod.rs and the Process methods in libpanda/src/process.rs. Done means SIGKILL terminates and can be waited on, SIGTERM arrives through HANDLE_PARENT, the listed kernel and userspace tests pass, and the opcode, variants, and docs/IPC.md are documented.