fix: tolerate stripped Excel language headers + observe background tasks + dep bumps

[logicallysynced]

Summary

Resolves an UnobservedTaskException reported by a downstream consumer (Chromatics, via Sentry) whose user runs a zh-TW Windows locale against what appears to be a CN-region FFXIV install. Lumina 7.4 rejects GetSheet<T>(Language.English) when the sheet's HeaderFile.Languages contains only Language.None; the throw fires inside MemoryHandler's fire-and-forget background task and was never observed, so the host AppDomain crashes at GC time on the default unhandled-task policy.

Two layers of fix, plus three bundled cleanups.

1. LuminaXivDatabaseProvider - tolerate missing per-language sheets

New private TryGetSheet<T>(GameData, Language, string) helper catches UnsupportedLanguageException and returns null. All three async methods now:

• Treat the iteration source as required: English Action/Status sheets, default-language TerritoryType. If unavailable, log one NLog warning and return without populating the dictionary - no throw.
• Treat JP/DE/FR (and the four PlaceName lookups for zones) as optional. Missing sheets contribute string.Empty to BuildLocalization instead of crashing.

BuildLocalization's four-string signature is preserved; CN/KR keep their Constants.UNKNOWN_LOCALIZED_NAME defaults.

2. MemoryHandler - observe the fire-and-forget tasks

Both Task.Run blocks in the constructor (resource resolution and signature resolution) now have .ContinueWith(OnlyOnFaulted) continuations that log the AggregateException via the existing Logger and raise OnException so consumers can surface it. Future exceptions in either background pipeline get observed and reported instead of becoming UnobservedTaskException at GC time.

3. Bundled in the same release

• FCS submodule bump 78d04f840 → e8908dbe5 (15 commits - Update structs ×3, AgentHUD UI display mode bitfield, RaptureShellModule.ChangeChatChannel return-type fix, InventoryContextEvent update, misc ghidra tooling refactors). No integrity-test pin drift.
• NLog + NLog.Schema bumped 6.1.2 → 6.1.3 (supersedes Dependabot PRs Bump NLog from 6.1.2 to 6.1.3 #109 and Bump NLog.Schema from 6.1.2 to 6.1.3 #110 - those can be closed).

Test plan

• dotnet test - 179/179 passing
• Debug build + deploy to Chromatics Build Dependencies (downstream consumer references the local DLL path)
• Live verify against the affected zh-TW / CN-region install once the consumer picks up the new DLL - expected behaviour: a single NLog warning per missing-language sheet, no crash

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