Show each GUI row's current reason and clear stale icons

[amrali-eg]

Problem

GUI result rows survive between runs.

• Stale icons: a file that failed, or was previewed as "would convert", and was then left unchanged or skipped in a later run kept the earlier run's icon, so the list described a run that was over (F4 from the 2026-09-20 review).
• No reason in the window: why a file failed, was refused or was left unchanged went only to Debug.WriteLine. The CSV export was the only place to read it; for example, "ASCII is already valid UTF-8 without a BOM." never appeared in the window.

Change

• UpdateResultItem clears the icon of a row left unchanged or skipped (ResultIconNone = -1).
• Every row's ToolTipText is its entry's current Diagnostic, set on each run and emptied when the run gives none, both for rows added by a scan and for rows updated by a conversion. lstResults.ShowItemToolTips = true.
• No column was added. CSV, journal and plan formats are unchanged.
• Backlog entry BL-41.

Tests

In PreviewResultPresentationTests:

• A theory: unchanged after a failed run, unchanged after a preview, skipped after a failed run. Each ends with no icon and an empty tooltip.
• The tooltip follows each result: the ASCII explanation, then an error's reason with the failed icon, then empty after a later success.
• A real MainForm has row tooltips on, and a scanned Validate row shows its reason with no icon.

Mutation checks (each restored byte-for-byte afterwards)

Mutation	Tests failing
Stale icon kept	3
Row tooltip not updated	4
Tooltips not turned on in the window	1
Scanned row has no tooltip	1

Verification

• Release build, 0 warnings; 980/980 tests pass.
• docs/Test-DefectBacklog.ps1 passes.
• GUI smoke on this build: all 10 phases passed, exit 0, no retries, Outcome Passed, managed assembly 13f752e2… matching the built DLL (evidence 20260928_211804). The smoke suite does not read icons or tooltips, so it confirms nothing else broke; no one has looked at the tooltip in a live window.

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