Expand adversarial evidence-path tests

[anusbutt]

Context

Evidence containment is security-sensitive across Unix and Windows/Git Bash path semantics. Existing traversal and symlink tests can be expanded with a table of platform-looking hostile references.

Scope

Add fixture-driven rejection cases for drive-letter forms, UNC-like paths, mixed separators, encoded-looking traversal, repeated separators, and valid nested POSIX paths.

This is a security-focused testing task. Keep evidence contents inert.

Acceptance criteria

• Each hostile shape has a rejecting test with the expected stable diagnostic.
• Valid nested repository-relative evidence remains accepted.
• Tests run in the existing Linux and Windows/Git Bash CI jobs.
• No test executes stored commands or follows an escaping symlink.