Harden replay path resolution

[AlexanderOnischenko]

Motivation

• Prevent replay code from reading or writing files outside the intended fixture roots by enforcing POSIX-relative paths and blocking escapes, absolute paths, parent traversal, and symlink-based escapes.
• Remove accidental non-hermetic behavior where base_dir is None allowed resolving arbitrary local paths.

Description

• Require base_dir in ReplayContext.resolve_resource_path and raise ValueError when it is not provided, and when an absolute resource_path is supplied.
• Add safe_join_under_validated(root, rel_path: Path) and keep safe_join_under(root, rel_posix: str) to avoid duplicate validation while still ensuring a joined path does not escape root.
• Update ReplayContext.resolve_resource_path to validate the POSIX-relative resource with validate_run_relative_posix and to use safe_join_under_validated for all resolved locations (direct resources/ paths and fixture lookups).
• Update copy_resource_files in src/fetchgraph/replay/export.py to validate and use safe_join_under_validated for both source (run_dir) and destination (out_dir) lookups to avoid writing outside expected roots.
• Export the new helper via __all__ in fetchgraph.utils.path_layout and clean up double-validation patterns.
• Add tests in tests/test_replay_runtime.py covering base_dir requirement, absolute path rejection, parent-traversal rejection, allowed inside-base resolution, and symlink escape rejection.

Testing

• Ran pytest tests/test_replay_runtime.py and all tests passed: 7 passed (no failures).

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Codex Task

[AlexanderOnischenko]

@codex, please review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

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