FileExtension/FullFileExtension throw ArgumentException for any path with a dot in a directory name ("./notes.txt", "/home/u/.config/a.json") #287

What's wrong

In Semantics.Paths/SemanticFilePath.cs:

• FileExtension (around line 14) searches the entire path with LastIndexOf('.').
• FullFileExtension (around line 53) does the same with IndexOf('.').

Neither limits the search to the file-name segment. When a directory component contains a dot, the substring handed to FileExtension.Create includes a directory separator. IsFileExtensionAttribute rejects that, so the property getter throws.

Failure scenarios (Linux, reproduced)

Paths with ./, ../, dot-directories such as .config, .git and .vscode, or versioned folder names are everyday input. For all of them, a simple property read throws. The existing tests use bare file names only (for example "test.tar.gz"), so the bug goes unnoticed.

Suggested fix

Work on the file-name segment only. Slice after the last / or \ (or use Path.GetFileName) before searching for dots. For FullFileExtension, also skip a leading dot in the file name, so .bashrc is treated as a name with no extension rather than an extension.

Acceptance criteria

• "./notes.txt" → FileExtension == ".txt" and FullFileExtension == ".txt".
• "../data/archive.tar.gz" → FileExtension == ".gz" and FullFileExtension == ".tar.gz".
• "/opt/app.v2/README" → both are empty.
• "/home/user/.bashrc" → both are empty, per the chosen dotfile rule, which should be documented.
• The tests cover both the netstandard2.0 branch and the span branch.