Match path containment case to the platform and accept paths directly under a root [patch]

[matt-edmondson]

Fixes #291
Fixes #282

Both issues are in the same prefix check, and both triage notes asked for them to be fixed together, so this is one PR.

What was wrong

• IsChildOf/IsParentOf ignore case on Linux: "/home/user/DOCS/report" is reported as inside "/home/user/docs" #291 (case): AbsoluteDirectoryPath.IsChildOf and AbsoluteFilePath.IsChildOf compared prefixes with OrdinalIgnoreCase on every platform. On Linux, /home/user/DOCS/report was reported as inside its sibling /home/user/docs, which defeats IsChildOf when it is used as a sandbox or output-directory guard. The "same path" branch also disagreed with the prefix branch: it was case-sensitive on the span targets and case-insensitive on netstandard2.0.
• IsChildOf/IsParentOf return false for every path directly under a filesystem root ("/home" vs "/", "C:\foo" vs "C:\") #282 (root): after the prefix match, the next character had to be a separator. A root (/, C:\) already ends in one, so /home, /etc.txt, and even /home/user were not children of /, although home.Parent is /.

Change

• New internal PathContainment helper, used by AbsoluteDirectoryPath.IsChildOf (and so IsParentOf) and AbsoluteFilePath.IsChildOf. This replaces three copies of the check, including the separate netstandard2.0 branch.
  • Comparison: OrdinalIgnoreCase on Windows, Ordinal everywhere else. The triage asked for an explicit decision on macOS: I chose Ordinal, the same comparison that path equality and hashing already use. APFS is case-insensitive by default but can be formatted case-sensitive, and ordinal is the choice that can't report a sibling as a child. The reasoning is written down in the helper's remarks.
  • Same path: the child must be strictly longer than the parent, so a path is never its own child. This also holds for a spelling that differs only in case.
  • Root: a parent that ends in a separator contains every longer path that starts with it. Otherwise the match must still end at a separator, so /home2 is not inside /home.
• Updated the XML docs, which described the old span-based implementation.

Tests

New PathContainmentTests, built from TestPaths so they run on every platform:

• A directory and a file under a differently cased sibling are children only on Windows. IsParentOf is checked too.
• A differently cased spelling of the same directory is never a child.
• A directory and a file directly under the root, and deeper paths, are children of the root. home.Parent == root is checked alongside.
• The root is not a child of itself.
• home2, home2/user, and home2.txt are not children of home.

Before the fix, 5 of the 8 tests failed on Linux: both case tests, and all three root tests. With the fix, the full Semantics.Test suite passes (1344 passed, 8 skipped), and Semantics.Paths builds for all five targets with no warnings.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PHrrj7bWzA48Gf3akqi476

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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