- Closes apple#1479.
- Using URL for filesystem paths is bad practice. FilePath is safer and
more ergonomic.

Replaces URL-based filesystem operations with SystemPackage.FilePath,
mirroring the pattern landed in apple#1480 for HostDNSResolver:

- configURL/anchorsURL → configPath/anchorsPath: FilePath
- defaultConfigPath/defaultAnchorsPath now FilePath
- FileManager calls switch from atPath: url.path to atPath: path.string
- Tests use String(contentsOfFile:) in place of String(contentsOf: URL)

Process.executableURL stays URL since Foundation.Process requires it.

- Closes #1517.
- Using URL for filesystem paths is bad practice. FilePath is safer and
more ergonomic.
- Sibling of #1480 (HostDNSResolver), same pattern.

Replaces URL-based path normalization with SystemPackage.FilePath in
the three Filesystem factory methods (block, volume, virtiofs):

- source: URL(fileURLWithPath: source).absolutePath() → FilePath(source).string

All existing callers (Parser, Utility, SnapshotStore, RuntimeConfigurationTests)
already pass absolute paths; the URL round-trip was a no-op in practice.
FilePath preserves separator normalization equivalent to the URL form.

Following the migration pattern from apple#1480 (HostDNSResolver),
apple#1518 (PacketFilter), and discussion apple#1481.

Replaces URL-based filesystem path handling with SystemPackage.FilePath,
following the pattern from apple#1480 (HostDNSResolver) and apple#1518 (PacketFilter):

- directoryURL → directoryPath: FilePath (public API)
- startWatching handler signature changes from ([URL]) to ([FilePath])
- LocalhostDNSHandler caller updated; the URL(filePath: configPath.string)
  wrapping shim is no longer needed
- FilePath.lastComponent (returns Optional<Component>) handled in
  LocalhostDNSHandler's filename filter

- Closes #1520.
- Using URL for filesystem paths is bad practice. FilePath is safer and
more ergonomic.
- Same pattern as #1480 (HostDNSResolver) and #1518 (PacketFilter).

- Closes #1521.
- Using URL for filesystem paths is bad practice. FilePath is safer and
more ergonomic.
- Same pattern as #1480 (HostDNSResolver) and #1518 (PacketFilter).

Converts RuntimeConfiguration.path, runtimeConfigurationPath, and the
readRuntimeConfiguration(from:) parameter from URL to FilePath. Following
the migration pattern from apple#1480 (HostDNSResolver), apple#1518 (PacketFilter),
and discussion apple#1481.

The two server-side callers (ContainersService, SandboxService) get
minimal URL <-> FilePath bridge calls. Bundle.create still requires URL
(Foundation API) so the bridge stays at that boundary.

Bundle+Log.swift in the same module is intentionally NOT converted in
this PR: all three of its callers use Foundation FileHandle APIs that
require URL, and converting bundle.containerLog without simultaneously
updating those callers (in ContainersService and SandboxService) would
expand the PR scope significantly. Bundle+Log can be converted later as
part of a SandboxService/ContainersService cluster conversion.

Convert PluginConfig.init from URL to FilePath, following the migration
pattern from apple#1480 (HostDNSResolver), apple#1518 (PacketFilter), and
discussion apple#1481.

- PluginConfig.init?(configURL: URL) → init?(configPath: FilePath)
- DefaultPluginFactory.findConfigURL → findConfigPath (returns FilePath?)
- Both internal call sites (DefaultPluginFactory, AppBundlePluginFactory)
  updated; bridge from URL to FilePath stays at the directory boundary
  since broader PluginFactory conversion is tracked separately.
- Tests updated to derive FilePath from temp URL.

- Closes CHAOS-1454.
- Internal cluster; URL boundary preserved at `directorySize` for `URLResourceKey.totalFileAllocatedSizeKey`.
- Sibling of apple#1480 (HostDNSResolver) and apple#1518 (PacketFilter), same pattern.

Converts RuntimeConfiguration.path, runtimeConfigurationPath, and the
readRuntimeConfiguration(from:) parameter from URL to FilePath. Following
the migration pattern from apple#1480 (HostDNSResolver), apple#1518 (PacketFilter),
and discussion apple#1481.

The two server-side callers (ContainersService, SandboxService) get
minimal URL <-> FilePath bridge calls. Bundle.create still requires URL
(Foundation API) so the bridge stays at that boundary.

Bundle+Log.swift in the same module is intentionally NOT converted in
this PR: all three of its callers use Foundation FileHandle APIs that
require URL, and converting bundle.containerLog without simultaneously
updating those callers (in ContainersService and SandboxService) would
expand the PR scope significantly. Bundle+Log can be converted later as
part of a SandboxService/ContainersService cluster conversion.

Convert PluginConfig.init from URL to FilePath, following the migration
pattern from apple#1480 (HostDNSResolver), apple#1518 (PacketFilter), and
discussion apple#1481.

- PluginConfig.init?(configURL: URL) → init?(configPath: FilePath)
- DefaultPluginFactory.findConfigURL → findConfigPath (returns FilePath?)
- Both internal call sites (DefaultPluginFactory, AppBundlePluginFactory)
  updated; bridge from URL to FilePath stays at the directory boundary
  since broader PluginFactory conversion is tracked separately.
- Tests updated to derive FilePath from temp URL.

- Closes CHAOS-1454.
- Internal cluster; URL boundary preserved at `directorySize` for `URLResourceKey.totalFileAllocatedSizeKey`.
- Sibling of apple#1480 (HostDNSResolver) and apple#1518 (PacketFilter), same pattern.

Convert PluginConfig.init from URL to FilePath, following the migration
pattern from apple#1480 (HostDNSResolver), apple#1518 (PacketFilter), and
discussion apple#1481.

- PluginConfig.init?(configURL: URL) → init?(configPath: FilePath)
- DefaultPluginFactory.findConfigURL → findConfigPath (returns FilePath?)
- Both internal call sites (DefaultPluginFactory, AppBundlePluginFactory)
  updated; bridge from URL to FilePath stays at the directory boundary
  since broader PluginFactory conversion is tracked separately.
- Tests updated to derive FilePath from temp URL.

- Closes CHAOS-1454.
- Internal cluster; URL boundary preserved at `directorySize` for `URLResourceKey.totalFileAllocatedSizeKey`.
- Sibling of apple#1480 (HostDNSResolver) and apple#1518 (PacketFilter), same pattern.

- Closes CHAOS-1457.
- Adds shared FilePath.absolute(_:relativeTo:) helper.
- Sibling of apple#1480 (HostDNSResolver) and apple#1518 (PacketFilter), same pattern.

Converts RuntimeConfiguration.path, runtimeConfigurationPath, and the
readRuntimeConfiguration(from:) parameter from URL to FilePath. Following
the migration pattern from apple#1480 (HostDNSResolver), apple#1518 (PacketFilter),
and discussion apple#1481.

The two server-side callers (ContainersService, SandboxService) get
minimal URL <-> FilePath bridge calls. Bundle.create still requires URL
(Foundation API) so the bridge stays at that boundary.

Bundle+Log.swift in the same module is intentionally NOT converted in
this PR: all three of its callers use Foundation FileHandle APIs that
require URL, and converting bundle.containerLog without simultaneously
updating those callers (in ContainersService and SandboxService) would
expand the PR scope significantly. Bundle+Log can be converted later as
part of a SandboxService/ContainersService cluster conversion.