Assertion failure in FilePath.lexicallyNormalized()

```console
% swift --version
Apple Swift version 6.2 (swift-6.2-RELEASE)
Target: arm64-apple-macosx26.0
Build config: +assertions
```

Using swift-system 1.6.3, attached file contains minimal reproducer.

Test code:

```swift
import SystemPackage
import Testing

@Test func pathNormalize() throws {
let path = FilePath("dir/../../../etc/pwned")
#expect(path.lexicallyNormalized() == "../../etc/pwned")
}
```

```console
% swift test
[1/1] Planning build
Building for debugging...
[1/1] Write swift-version--2028C2505FF3CBA9.txt
Build complete! (0.13s)
Test Suite 'All tests' started at 2026-01-11 13:48:30.341.
Test Suite 'All tests' passed at 2026-01-11 13:48:30.342.
Executed 0 tests, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
􀟈 Test run started.
􀄵 Testing Library Version: 6.2 (3fdabe5392108d8)
􀄵 Target Platform: arm64-apple-macosx
􀟈 Test pathNormalize() started.
SystemPackage/FilePathParsing.swift:231: Assertion failed
error: Exited with unexpected signal code 5
```

[20260111-lexicalnormalize-bug.tgz](https://github.com/user-attachments/files/24555154/20260111-lexicalnormalize-bug.tgz)

Start with FilePathParsing.swift at the assertion reported on line 231, then run the pathNormalize reproducer from the issue with Swift 6.2 and swift-system 1.6.3. Trace lexicallyNormalized() for "dir/../../../etc/pwned" and add or update the relevant test so it produces "../../etc/pwned" without an assertion failure.

Written by the indexing model from the issue text.