1/ Add macOS and Linux implementations of a Thread Count gauge
2/ On macOS, uses `ProcessTaskInfo` API
3/ On Linux, uses `sysfs`
4/ Zombie threads are counted in both cases (reason: resources are in use,
   and if not leaked those should be observer to decrement anyways, soon
   matching what's "runnable", but if not, these alert of the leak)
5/ Adds tests, using XCTest to benefit from its default process isolation
   (harder to test these with Swift Testing)
6/ Note that on macOS, tests don't reliably see resource clean-up without
   even less reliable waits. So test gauge going up and down in Linux, but
   only going up on macOS.

Tests:

$ swift-format lint -r --strict .

$ uname && xcrun swift test --enable-all-traits
Darwin
[...]
Test Suite 'swift-system-metricsPackageTests.xctest' started at 2026-02-23 19:12:57.942.
Test Suite 'ThreadCountTests' started at 2026-02-23 19:12:57.942.
Test Case '-[SystemMetricsTests.ThreadCountTests test_threadCount]' started.
Test Case '-[SystemMetricsTests.ThreadCountTests test_threadCount]' passed (0.000 seconds).
Test Suite 'ThreadCountTests' passed at 2026-02-23 19:12:57.943.
         Executed 1 test, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
Test Suite 'swift-system-metricsPackageTests.xctest' passed at 2026-02-23 19:12:57.943.
         Executed 1 test, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
Test Suite 'All tests' passed at 2026-02-23 19:12:57.943.
         Executed 1 test, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
◇ Test run started.
[...]
✔ Test run with 16 tests in 3 suites passed after 2.009 seconds.

$ uname && swift test --enable-all-traits
Linux
[...]
Test Suite 'debug.xctest' started at 2026-02-24 03:15:37.877
Test Suite 'ThreadCountTests' started at 2026-02-24 03:15:37.877
Test Case 'ThreadCountTests.test_threadCount' started at 2026-02-24 03:15:37.877
Test Case 'ThreadCountTests.test_threadCount' passed (0.018 seconds)
Test Suite 'ThreadCountTests' passed at 2026-02-24 03:15:37.896
         Executed 1 test, with 0 failures (0 unexpected) in 0.018 (0.018) seconds
Test Suite 'debug.xctest' passed at 2026-02-24 03:15:37.896
         Executed 1 test, with 0 failures (0 unexpected) in 0.018 (0.018) seconds
Test Suite 'All tests' passed at 2026-02-24 03:15:37.896
         Executed 1 test, with 0 failures (0 unexpected) in 0.018 (0.018) seconds
◇ Test run started.
[...]
✔ Test run with 13 tests in 3 suites passed after 1.269 seconds.

1/ Breaking changes. Didn't notice these are package and not internal,
   not sure what's the best approach for keeping up stuff like label
   literals in the intended design or stuff like init deprecation in
   Swift library best-practices. PTAL.
2/ 6.0 breakages: seem to differ from swift-format lint enforced ones,
   cleaned up.
3/ SemVer label: I do not seem to have permission to label.

4/ Removed macOS tests for thread count.
5/ Reworded comments around the added use of XCTest

Across mac/linux, all clean:

```
$ swift-format lint -r --strict .
$ swift test -Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable
$ swift package diagnose-api-breaking-changes main
```

Missed staging... just protecting #import FoundationEssentials
regardless of being in conditionally compiled code for glibc/musl.

Tests:

On mac and linux, clean:

```
$ swift-format lint -r --strict .
$ swift test -Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable
```

Tests:

$ swift-format lint -r --strict .
$ swift test -Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable
✔ Test run with 13 tests passed after 1.627 seconds.

1/ Remove the OS &run-isolation dependent thread tests
2/ Add missing basic test per feedback

*Tests*:

On MacOS and Linux.

$ swift-format lint -r --strict .

$ swift test -Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable
[...]
✔ Test run with 16 tests in 3 suites passed after 2.013 seconds.

$ swift package diagnose-api-breaking-changes main

2 breaking changes detected in SystemMetrics:
  💔 API breakage: constructor SystemMetricsMonitor.Data.init(virtualMemoryBytes:residentMemoryBytes:startTimeSeconds:cpuSeconds:maxFileDescriptors:openFileDescriptors:) has been removed
  💔 API breakage: constructor SystemMetricsMonitor.Configuration.Labels.init(prefix:virtualMemoryBytes:residentMemoryBytes:startTimeSeconds:cpuSecondsTotal:maxFileDescriptors:openFileDescriptors:) has been removed

*Note*: Only the expected ones, to be overridden to merge.