Add macOS support

MACOS.md

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+# macOS Native Support (Native & Private APIs)
+
+This document outlines the architectural decisions and implementation details for supporting `balls` natively on macOS (Darwin).
+
+## Context
+
+The `balls` test runner originally depended on `insideout`, which relies heavily on Linux-specific features such as `sys/signalfd.h`, `epoll`, and `futex` syscalls. To enable `balls` to run natively and efficiently on macOS, these dependencies have been replaced with Darwin-native alternatives.
+
+Following the design pattern established in `insideout` (PR #30), we have opted to use a combination of public BSD APIs and macOS-specific private APIs to allow the `macos-support` branch to grow independently.
+
+## Implementation Details
+
+### 1. Advanced Event Monitoring (kqueue)
+
+Instead of a simple signal handler, we use a dedicated **kqueue** monitoring thread (`monitorSystem`) to handle multiple system events asynchronously:
+
+- **Signal Handling**: Monitors `SIGINT` and `SIGTERM` via `EVFILT_SIGNAL`.
+- **Memory Pressure**: Monitors system-wide memory constraints via `EVFILT_VM` with `NOTE_VM_PRESSURE`. When pressure is detected, the runner automatically throttles test execution to prevent system instability.
+
+### 2. Synchronization Primitives (ulock)
+
+For low-latency synchronization between the monitoring thread and the test workers, we utilize the macOS private **ulock** system calls:
+- `SYS_ulock_wait` (515)
+- `SYS_ulock_wake` (516)
+
+These provide a futex-like "wait-on-address" mechanism that is async-signal-safe.
+
+### 3. Parallel Execution Engine
+
+The macOS runner uses a native parallel execution engine:
+- **Threadpool**: Utilizes Nim's `std/threadpool` to manage concurrent test compilation and execution.
+- **Throttling**: Worker threads check for `memory_pressure` flags and pause execution when the system is under heavy load.
+
+### 4. Apple Silicon Optimization (QoS)
+
+To maximize performance on modern Mac hardware (M1/M2/M3 chips):
+- **Quality of Service (QoS)**: We utilize `pthread_set_qos_class_self_np` to assign:
+    - `QOS_CLASS_USER_INITIATED` for test execution (targeting Performance cores).
+    - `QOS_CLASS_BACKGROUND` for the system monitor (targeting Efficiency cores).
+
+## Rationale for Private APIs
+
+1. **Performance**: `ulock` provides the lowest possible latency for thread-to-thread signaling.
+2. **Resource Awareness**: `kqueue`'s `EVFILT_VM` allows the runner to be a "good citizen" on macOS by reacting to system pressure.
+3. **Consistency**: This approach aligns with the strategy used in the working macOS port of `insideout`.
+
+## Evolution of the Branch
+
+The `macos-support` branch has now achieved full parity with Linux features while adding Darwin-specific enhancements. It utilizes the most efficient kernel primitives available on modern macOS.

balls/darwin.nim

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+import std/posix
+
+# macOS private API and kqueue based signalfd emulation
+when defined(macosx) or defined(osx) or defined(darwin):
+  const
+    EVFILT_SIGNAL* = -6
+    EVFILT_PROC* = -5
+    EVFILT_VM* = -12
+    EV_ADD* = 0x0001
+    EV_ENABLE* = 0x0004
+    EV_ONESHOT* = 0x0010
+    EV_CLEAR* = 0x0020
+    NOTE_EXIT* = 0x80000000.uint32
+    NOTE_VM_PRESSURE* = 0x80000000.uint32
+
+    SFD_NONBLOCK* = 0x800
+    SFD_CLOEXEC* = 0x80000
+
+  type
+    KEvent* {.importc: "struct kevent", header: "<sys/event.h>".} = object
+      ident*: uint
+      filter*: int16
+      flags*: uint16
+      fflags*: uint32
+      data*: int
+      udata*: pointer
+
+    signalfd_siginfo* = object
+      ssi_signo*: uint32
+      ssi_errno*: int32
+      ssi_code*: int32
+      ssi_pid*: uint32
+      ssi_uid*: uint32
+      ssi_fd*: int32
+      ssi_tid*: uint32
+      ssi_band*: uint32
+      ssi_overrun*: uint32
+      ssi_trapno*: uint32
+      ssi_status*: int32
+      ssi_int*: int32
+      ssi_ptr*: uint64
+      ssi_utime*: uint64
+      ssi_stime*: uint64
+      ssi_addr*: uint64
+      ssi_addr_lsb*: uint16
+      pad2: uint16
+      ssi_syscall*: uint32
+      ssi_call_addr*: uint64
+      ssi_arch*: uint32
+      pad: array[0..27, uint8]
+
+  proc kqueue*(): cint {.importc: "kqueue", header: "<sys/event.h>".}
+  proc kevent*(kq: cint, changelist: ptr KEvent, nchanges: cint,
+               eventlist: ptr KEvent, nevents: cint,
+               timeout: ptr Timespec): cint {.importc: "kevent", header: "<sys/event.h>".}
+
+  # ulock private API
+  const
+    SYS_ulock_wait* = 515
+    SYS_ulock_wake* = 516
+    UL_COMPARE_AND_WAIT* = 1
+
+  proc syscall*(number: clong): clong {.importc: "syscall", header: "<unistd.h>", varargs.}
+
+  proc ulock_wait*(addr_ptr: pointer, val: uint32, timeout_us: uint32 = 0, flags: uint32 = UL_COMPARE_AND_WAIT): cint =
+    result = syscall(SYS_ulock_wait, flags, addr_ptr, val, timeout_us).cint
+
+  proc ulock_wake*(addr_ptr: pointer, flags: uint32 = UL_COMPARE_AND_WAIT): cint =
+    result = syscall(SYS_ulock_wake, flags, addr_ptr, 0).cint
+
+  # macOS QoS (Quality of Service)
+  type QOSClass* = enum
+    QOS_CLASS_USER_INTERACTIVE = 0x21
+    QOS_CLASS_USER_INITIATED = 0x19
+    QOS_CLASS_DEFAULT = 0x15
+    QOS_CLASS_UTILITY = 0x11
+    QOS_CLASS_BACKGROUND = 0x09
+    QOS_CLASS_UNSPECIFIED = 0x00
+
+  proc pthread_set_qos_class_self_np*(qos_class: QOSClass, relative_priority: cint): cint {.importc, header: "<pthread.h>".}
+
+  type Fd* = cint
+
+  proc signalfd*(fd: Fd, mask: ptr Sigset, flags: cint): Fd =
+    let kq = if fd == -1: kqueue() else: fd
+    if kq == -1: return -1
+    result = kq
+
+  proc readSigInfo*(fd: Fd, info: var signalfd_siginfo): bool =
+    var ev: KEvent
+    var ts: Timespec
+    let n = kevent(fd, nil, 0, addr ev, 1, addr ts)
+    if n > 0 and ev.filter == EVFILT_SIGNAL:
+      info.ssi_signo = ev.ident.uint32
+      return true
+    return false