Use SystemPath for DirectoryWatcher

[chrisgeo]

Summary

Converts DirectoryWatcher from URL to SystemPackage.FilePath, mirroring the pattern landed in apple#1480 (HostDNSResolver) and apple#1518 (PacketFilter).

Changes

• directoryURL: URL → directoryPath: FilePath (public API)
• init(directoryURL:, log:) → init(directoryPath:, log:)
• startWatching(handler:) closure signature: ([URL]) → ([FilePath])
• LocalhostDNSHandler.swift caller updated; the URL(filePath: configPath.string) wrapping shim is no longer needed

Behavioral note

URL.lastPathComponent returns String; FilePath.lastComponent returns FilePath.Component? (optional). The filename filter in LocalhostDNSHandler handles the optional explicitly via $0.lastComponent?.string.starts(with: ...) == true. This preserves the prior "skip malformed entries" behavior.

Context

• Upstream tracking: apple/container discussion Good first issues - switching from URL to FilePath. apple/container#1481 (URL → FilePath migration)
• Reference PRs: Use SystemPath for HostDNSResolver. apple/container#1480 (HostDNSResolver), Use SystemPath for PacketFilter. apple/container#1518 (PacketFilter)
• Internal: CHAOS-1450

Test plan

• swift test --filter DirectoryWatcherTest — all tests pass
• swift test --filter ContainerOSTests — full module passes (no neighbor regressions)
• swift-format strict lint clean
• Full project builds

[linear]

CHAOS-1450