fix: add cross-platform support for disk space checks

Split diskspace.go into platform-specific files:
- diskspace_unix.go for Unix/Linux/macOS
- diskspace_windows.go for Windows

This fixes the release build failure where Windows builds were
failing due to undefined syscall.Statfs (Unix-only).

Author: malston

pkg/pivnet/diskspace.go pkg/pivnet/diskspace_unix.gopkg/pivnet/diskspace.go renamed to pkg/pivnet/diskspace_unix.go

@@ -1,3 +1,5 @@
+//go:build unix
+
 // ABOUTME: Disk space checking and management utilities.
 // ABOUTME: Ensures sufficient disk space before downloads with configurable buffer.
 package pivnet

pkg/pivnet/diskspace_windows.go

@@ -0,0 +1,111 @@
+//go:build windows
+
+// ABOUTME: Disk space checking and management utilities for Windows.
+// ABOUTME: Ensures sufficient disk space before downloads with configurable buffer.
+package pivnet
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// DiskManager handles disk space checks
+type DiskManager struct {
+	minFreeSpaceGB int64
+}
+
+// NewDiskManager creates a new disk space manager
+func NewDiskManager(minFreeSpaceGB int64) *DiskManager {
+	return &DiskManager{
+		minFreeSpaceGB: minFreeSpaceGB,
+	}
+}
+
+// HasEnoughSpace checks if there's enough disk space for a download
+func (d *DiskManager) HasEnoughSpace(path string, fileSize int64) (bool, error) {
+	// If path doesn't exist, check parent directory
+	checkPath := path
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		checkPath = filepath.Dir(path)
+		// Keep going up until we find an existing directory
+		for {
+			if _, err := os.Stat(checkPath); err == nil {
+				break
+			}
+			parent := filepath.Dir(checkPath)
+			if parent == checkPath {
+				// Reached root without finding existing dir
+				return false, fmt.Errorf("no existing directory found in path hierarchy")
+			}
+			checkPath = parent
+		}
+	}
+
+	availableBytes, err := d.getAvailableSpaceWindows(checkPath)
+	if err != nil {
+		return false, err
+	}
+
+	// Required space: file size + minimum buffer
+	requiredBytes := d.calculateRequired(fileSize)
+
+	return availableBytes >= uint64(requiredBytes), nil
+}
+
+// GetAvailableSpace returns available space in bytes
+func (d *DiskManager) GetAvailableSpace(path string) (int64, error) {
+	// If path doesn't exist, check parent directory
+	checkPath := path
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		checkPath = filepath.Dir(path)
+		for {
+			if _, err := os.Stat(checkPath); err == nil {
+				break
+			}
+			parent := filepath.Dir(checkPath)
+			if parent == checkPath {
+				return 0, fmt.Errorf("no existing directory found in path hierarchy")
+			}
+			checkPath = parent
+		}
+	}
+
+	availableBytes, err := d.getAvailableSpaceWindows(checkPath)
+	if err != nil {
+		return 0, err
+	}
+
+	return int64(availableBytes), nil
+}
+
+// getAvailableSpaceWindows uses Windows API to get disk space
+func (d *DiskManager) getAvailableSpaceWindows(path string) (uint64, error) {
+	var freeBytesAvailable, totalBytes, totalFreeBytes uint64
+
+	pathPtr, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, fmt.Errorf("failed to convert path: %w", err)
+	}
+
+	err = windows.GetDiskFreeSpaceEx(
+		pathPtr,
+		(*uint64)(unsafe.Pointer(&freeBytesAvailable)),
+		(*uint64)(unsafe.Pointer(&totalBytes)),
+		(*uint64)(unsafe.Pointer(&totalFreeBytes)),
+	)
+	if err != nil {
+		return 0, fmt.Errorf("failed to check disk space: %w", err)
+	}
+
+	return freeBytesAvailable, nil
+}
+
+// calculateRequired calculates total space needed (file + buffer)
+func (d *DiskManager) calculateRequired(fileSize int64) int64 {
+	bufferBytes := d.minFreeSpaceGB * 1024 * 1024 * 1024
+	return fileSize + bufferBytes
+}