Align syscall documentation, source declarations, and contracts

[Pummelchen]

Summary

Reconcile the syscall ABI across current source, userspace headers, API docs, QEMU contract data, and gate expectations.

Problem / objective

Current source exposes syscalls through XAIOS_SYSCALL_AGENT_DISPATCH number 34, while contracts/qemu-rc-v1.json covers only 1-28 and docs/gate markers may lag newer socket and agent-dispatch paths.

Acceptance criteria

• Inspect current syscall declarations and dispatch table.
• Update or explicitly justify docs/API.md, contracts/qemu-rc-v1.json, and QEMU gate expectations.
• Ensure userspace wrapper declarations remain aligned with kernel ABI.
• Record any intentional contract freeze boundaries.

Validation

• make compile-check
• make qemu-abi-contract
• make qemu-smoke if QEMU/toolchain is available

Relevant files

• kernel/include/xaios/syscall.h
• kernel/user/syscall.c
• userspace/include/xaios_user.h
• docs/API.md
• contracts/qemu-rc-v1.json
• scripts/qemu-smoke.py
• scripts/qemu_gate_lib.py