Fix compressor skip-list filtering

[marcmy]

Summary

• fix the compressor work-list filter so entries explicitly marked as non-compressible are actually excluded
• handle both extension entries (for example .mp4) and exact file paths in the same skip set
• make skip matching case-insensitive, consistent with Windows path semantics

Root cause

The current predicate combines extension and exact-path checks with this condition:

((fl.FileInfo != null && !excludedFileExtensions.Contains(fl.FileInfo.Extension))
    || excludedFileExtensions.Contains(fl.FileName))

That final || excludedFileExtensions.Contains(fl.FileName) is inverted: an exact file path in the exclusion list makes the predicate true instead of excluding the file. The set is also case-sensitive.

This matters specifically for the previously determined per-file poor-compression list: CompactGUI passes those entries to the compactor as exact full paths, so the old predicate could make the very file that was supposed to be skipped eligible for compression.

Expected behavior

A file is eligible only when:

• it is not already using the selected compression mode
• it is not excluded by a resume checkpoint
• it is larger than the filesystem cluster size
• its extension is not in the configured non-compressible extension list
• its exact path is not in the folder/wiki poor-compression skip list

Validation

• supply-chain audit for this PR: passed
• the exact combined source tree containing this fix plus Fix manual compression resume lifecycle #24/Start CompactGUI UI overhaul foundation #26/Replace slate-blue UI with charcoal theme #27 was also run through the Windows x64 build workflow: restore, self-contained publish, packaging, and artifact upload all passed

Runtime checks

1. Enable the built-in poorly-compressed file-type skip option and verify known excluded extensions do not appear in the compression work list.
2. Enable the user/wiki poor-compression skip option and verify exact file paths identified as poor candidates do not appear in the work list.
3. Repeat with path/extension casing differences and verify exclusions remain case-insensitive.
4. Disable the relevant skip option and verify those files can become eligible again.