Record deferred findings and known test coverage gaps

[amrali-eg]

Docs only (docs/DEFECT-BACKLOG.md).

• BL-42 (Open, Low, Common): a preview's CSV report calls files Converted. Left open for a decision on the wording of a scripted contract.
• BL-43 (Open, Low, Rare): a file name with an unpaired UTF-16 surrogate cannot be planned or backed up, because JSON replaces the unpaired half with U+FFFD. Reproduced on v3.14.5; the source is never modified, but the failure arrives late and names the wrong cause.
• Known test coverage gaps, a new section outside the ledger, each checked against master at 23d83c4: settings loading and mask parsing, the converter's link and unknown-installation branches, the Ctrl+C handlers, and the self-contained executable.
• Withdraws an earlier claim that the GUI smoke report hashes only the launcher; it also records the managed assembly's hash.

docs/Test-DefectBacklog.ps1 passes (87 findings: 75 fixed, 8 open).

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