RelativeDirectoryPath.Normalize() drops leading "..": "../sibling" becomes "sibling", so the path points somewhere else

[matt-edmondson]

What's wrong

RelativeDirectoryPath.Normalize() (Semantics.Paths/Implementations/RelativeDirectoryPath.cs, around line 218) normalizes a relative path in three steps:

1. Combine it with a dummy root (/ or C:\).
2. Call Path.GetFullPath.
3. Take GetRelativePath back from the dummy root.

A .. at the filesystem root resolves to the root itself, so every .. that climbs above the starting point is silently discarded.

Failure scenario (reproduced)

Input	Normalize() returns	Expected
"../sibling"	"sibling"	"../sibling"
"a/../../b"	"b"	"../b"
"../../x/y"	"x/y"	"../../x/y"

Take "../sibling" resolved from /home/user/project. Before normalizing it names /home/user/sibling. After normalizing it names /home/user/project/sibling. Normalization must never change which directory a path refers to. Here it can turn a path that points outside the base into one that points inside it, which affects file operations and makes any containment or sandbox check built on it unreliable.

The existing test Normalize_WithOnlyDots_ResolvesCorrectly (./../folder) only asserts that the result contains "folder", so it passes despite the bug.

Suggested fix

Resolve segments lexically with a stack:

• skip .
• a .. pops the previous segment when there is one that isn't itself ..
• otherwise keep the ..

Then join with the platform separator. This needs no dummy base and gives the same result on every OS.

Acceptance criteria

• The three inputs in the table above normalize to the expected column.
• "a/./b/../c" → "a/c".
• Normalize_WithOnlyDots_ResolvesCorrectly asserts the exact value ("../folder").

[matt-edmondson]

Triage

• Category: Bug
• Priority: High. Normalize() changes which directory a path refers to (../sibling → sibling), and it does so silently. Any containment or sandbox check built on the normalized value can be bypassed or give wrong answers.
• Area / suggested assignee: Semantics.Paths — RelativeDirectoryPath.Normalize() (dummy-root + GetFullPath round trip). Owner: @matt-edmondson
• Duplicates / in progress: None found. No open PR touches path normalization. Related but separate: FileExtension/FullFileExtension throw ArgumentException for any path with a dot in a directory name ("./notes.txt", "/home/u/.config/a.json") #287 (extension parsing in the same Semantics.Paths package), which could ship in the same patch release.
• Next step: Replace the dummy-root round trip with a lexical segment stack as proposed. Tighten Normalize_WithOnlyDots_ResolvesCorrectly to assert "../folder" exactly. Also check whether other Normalize() implementations (relative file paths, for example) use the same dummy-root trick.

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