feat: add registry resource type for Windows

[Blankf]

Depends on: #1055 (fix: bytes.Reader issue in HavePatternsMatcher)

Summary

Adds a new registry resource type that validates Windows registry keys natively using the golang.org/x/sys/windows/registry API. This replaces the need to shell out to PowerShell for registry checks.

Closes #616

Motivation

The ansible-lockdown Windows CIS audit repos (Windows 2016/2019/2025) currently rely entirely on the command resource shelling out to PowerShell for ~300+ registry checks per benchmark. This is slow (~180ms per check due to PowerShell process spawn) and verbose.

With the native registry resource, the same checks run in milliseconds via direct Windows API calls.

Benchmark results (250 identical checks):

Method	Time
Native registry resource	0.020s
command + PowerShell	43.784s

Gossfile syntax

registry:
  HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProductName:
    exists: true
    value: "Windows Server 2025 Datacenter"
    type: REG_SZ

For value names containing backslashes (e.g. HardenedPaths UNC entries), use :: as an explicit separator:

registry:
  HKLM\SOFTWARE\Policies\Microsoft\Windows\NetworkProvider\HardenedPaths::\\*\NETLOGON:
    exists: true
    value: "RequireMutualAuthentication=1, RequireIntegrity=1"

Details

• Supported hives: HKLM, HKCU, HKCR, HKU, HKCC
• Supported types: REG_SZ, REG_EXPAND_SZ, REG_DWORD, REG_QWORD, REG_BINARY, REG_MULTI_SZ
• Opens keys with QUERY_VALUE access (read-only, least privilege)
• On non-Windows platforms, returns an error ("registry resource is only supported on Windows"), following the NullPackage pattern
• Cross-compilation from Linux continues to work

Files changed

New files:

• system/registry.go -- Registry interface + key path parser
• system/registry_test.go -- 17 unit tests for the parser
• system/registry_notwindows.go -- Non-Windows stub
• system/registry_windows.go -- Windows implementation
• resource/registry.go -- Resource type definition
• integration-tests/goss/windows/tests/registry.goss.yaml -- Integration tests
• docs/windows-parity-progress.md -- Progress tracker

Modified files:

• system/system.go -- Added NewRegistry factory
• resource/resource_list_genny.go + resource/resource_list.go -- Added RegistryMap
• goss_config.go -- Wired into config
• add.go -- Added to AddResource switch
• cmd/goss/goss.go -- Added CLI subcommand
• docs/schema.yaml -- Added schema definition
• docs/platforms.md -- Updated platform matrix

Test plan

• go vet ./... passes
• go test ./... passes (17 new parser tests + all existing)
• GOOS=windows GOARCH=amd64 go build cross-compiles successfully
• gosec ./system/ ./resource/ -- zero findings in new code
• Tested on Windows Server 2025 -- registry checks work correctly
• Tested on Windows Server 2025 -- existing command resource still works
• Benchmarked: 250 registry checks in 0.020s vs 43.784s via PowerShell