bugfix #1925: stop library scans from silently dropping titles

[rmcrackan]

Diagnoses and addresses #1925, where a title visible on Audible did not appear in Libation and left no useful trace in the log. Also fixes an unrelated test-suite bottleneck found while working on it.

Diagnosis

The attached log contains 1140 scans. The direct library page count is stable, but podcast expansion is not:

Library items returned by Audible	Post-expansion totals observed
434	2141, 2142, 2143
438	2145, 2146
439	2147
440	2147, 2148, 2149, 2150, 2151

1117 consecutive scans read exactly 440 direct library items but produced five different expanded totals. Audible's catalog endpoint can return HTTP 200 while omitting requested products. Libation accepted that partial response, then silently removed episodes that could not be linked to a series parent. Separately, an episode already in the database whose parent was absent rendered nowhere, because neither grid treated it as a product or as a displayable series child.

The log could not explain other exclusions either: orphan pruning was Debug-only and did not name titles, and ImportEpisodes / ImportPlusTitles were not logged at all.

Scan changes

• Compare every successful catalog response against the requested ASINs.
• Re-request omitted ASINs (2 extra attempts), then warn with any Audible still will not return.
• Warn with ASIN and title when an episode is dropped for having no series parent.
• Emit a per-scan tally: direct items, expanded episodes, dropped orphans, filter settings, filter exclusions, final import count.
• Record ImportEpisodes, ImportPlusTitles, and DownloadEpisodes in the startup state block.
• Read the import filters once per scan so a mid-scan settings change cannot half-filter a library.

Grid changes

• Show database orphan episodes as standalone rows in both Avalonia and WinForms.
• Move a standalone orphan under its parent once a later scan finds it, and back to a standalone row if the parent disappears.

How this addresses #1925

Three protections at the points where the reported title could vanish:

1. Audible omits an episode or parent from a successful catalog response -> Libation re-requests that exact ASIN instead of accepting the gap.
2. The parent stays unavailable -> the log names the excluded episode and the reason.
3. The episode is already stored but its parent is absent -> both grids show it as a standalone row, so it stays searchable and downloadable.

A non-episode title omitted from Audible's direct paged library response still cannot be recovered, since Libation never receives an ASIN for it. The new tally makes that API-side discrepancy visible and distinguishes it from local filtering.

Test suite bottleneck (unrelated to #1925, found while testing)

IdentityTokenStorageWiring.ConfigureFrom always calls ResolveSecretStore, which falls through to OsSecretStore.Create(...).IsAvailable when no master key file or env var is set. That is a blocking libsecret call. On a headless machine, or one whose login keyring is locked, it waits on a desktop unlock prompt that never gets answered. Five tests reached it and one reached it twice. Guarding with an availability probe does not help, because the probe is the blocking call.

• Tests that only assert which write method gets configured now resolve the master key from a temp key file, so they short-circuit before the OS store. This also stops them minting a last-resort key file into the real Libation folder.
• The two tests that exist to exercise the real OS store are opt-in via LIBATION_TEST_OS_SECRET_STORE=1.
• AGENTS.md corrected: it previously said to wait for a human to set a keyring password, which does not work here.

AudibleUtilities.Tests went from 12m 20s to 1.8s. The test that used to fail in this environment is now a clean skip.

Verification

Avalonia and CLI builds pass with 0 errors. All Linux-buildable test projects, each under a hard timeout:

AudibleUtilities       total: 74  failed: 0  succeeded: 72  skipped: 2   duration: 1s 842ms
ApplicationServices    total: 46  failed: 0  succeeded: 46               duration:    363ms
FileLiberator          total:  8  failed: 0  succeeded:  8               duration:    395ms
FileManager            total: 202 failed: 0  succeeded: 202              duration: 1s 299ms
LibationFileManager    total: 624 failed: 0  succeeded: 603              duration: 1s 304ms
LibationSearchEngine   total: 40  failed: 0  succeeded: 40               duration:    327ms
LibationUiBase         total: 32  failed: 0  succeeded: 32               duration:    412ms

LibationUiBase includes direct verification that an orphaned episode becomes a standalone LibraryBookEntry. Zero failures anywhere; the whole suite now runs in about 6 seconds.