fix: Windows cross-compilation support (GOOS=windows)

- debugcli.go/debugtui.go: add //go:build !windows tag
- debugcli_windows.go/debugtui_windows.go: no-op stubs
- cdx/cdx.go: extract mmap to platform-specific files
- cdx/mmap_posix.go: syscall.Mmap/Munmap
- cdx/mmap_windows.go: no-op (falls back to read)
- ntx/ntx.go, ntx/build.go: same mmap extraction
- ntx/mmap_posix.go, ntx/mmap_windows.go: platform split

Builds verified: linux/amd64, windows/amd64, darwin/arm64, darwin/amd64

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hbrt/debugcli.go

@@ -1,3 +1,5 @@
+//go:build !windows
+
 // Copyright (c) 2026 Charles KWON OhJun (charleskwonohjun@gmail.com)
 // All rights reserved.
 

hbrt/debugcli_windows.go

@@ -0,0 +1,13 @@
+//go:build windows
+
+package hbrt
+
+import "fmt"
+
+// CLIDebugger returns a no-op debug callback on Windows.
+func CLIDebugger() DebugCallback {
+	return func(event *DebugEvent) int {
+		fmt.Println("[debugger not available on Windows]")
+		return 0 // continue
+	}
+}

hbrt/debugtui.go

@@ -1,3 +1,5 @@
+//go:build !windows
+
 // Copyright (c) 2026 Charles KWON OhJun (charleskwonohjun@gmail.com)
 // All rights reserved.
 

hbrt/debugtui_windows.go

@@ -0,0 +1,13 @@
+//go:build windows
+
+package hbrt
+
+import "fmt"
+
+// TUIDebugger returns a no-op debug callback on Windows.
+func TUIDebugger() DebugCallback {
+	return func(event *DebugEvent) int {
+		fmt.Println("[TUI debugger not available on Windows]")
+		return 0 // continue
+	}
+}