Phase 3: extract pure cores + inject functions (functional refactor plan)

[jaybarden1]

Summary

Phase 3 of docs/wallpaper-scrapper-functional-refactor-plan.md: extract pure, unit-testable cores from the page/workflow shells and inject effectful dependencies. Page public APIs unchanged — Result-chain conversion is Phase 4.

Pure cores (frozen behaviour, quirks preserved)

• PageHeaderParser, SubscriptionHeaderParser, TopWallpapersHeaderParser — string? → Result<PageInfo|int, ScrapeError>; shells Match back to existing tuple returns
• ImageLinkSelector.SelectWanted — pure href filter (non-null, contains /w/, Take 24)
• TagRules.Evaluate — pure port of ImagePage.ProcessTheImageTagsAsync predicates via TagRuleContext → TagOutcome DU (SkipImage | Accept); DB save stays in the shell
• SearchProgress record + SearchProgressFunctions pure state transitions for SearchWorkflow

New error model

• ScrapeError DU (7 cases) + ScrapeErrorFactory + LogFailure Serilog extension (plan §4.5)

Injected effects

• IDelayStrategy / RandomDelayStrategy (all delay sites: category up-to-date, page navigation, already-downloaded, before-image, retry)
• IImageRetriever / ImageRetriever (HttpClient, returns Result<byte[], ScrapeError>) — replaces deleted static ImageRetrieverHelper
• IImageSaver / ImageSaver (IFileSystem) — replaces deleted static ImageSaveHelper
• DirectoryHelper now takes IFileSystem; TimeProvider replaces Stopwatch in SearchWorkflow
• DI: IDelayStrategy singleton, IImageSaver transient, AddHttpClient<IImageRetriever, ImageRetriever> (2-min timeout)

Documented deviations (reviewed & accepted)

• Empty (non-null) header now short-circuits to PageParseFailed instead of throwing a framework exception inside the parse logic; callers' log-and-rethrow behaviour unchanged (pinned by tests)
• Failed image download now throws InvalidOperationException (was effectively FileNotFoundException via silent-empty-array + missing file); retry-once flow unchanged (pinned by tests)
• Latent [^50..] crash in ImagePageService logging fixed with a length guard
• Test csproj gains a global using for the Services namespace (sibling-namespace visibility)

Test plan

• TDD: red tests committed first (aa3e01d), implementation after
• Full suite green: 239 Wallpaper.Scrapper (incl. all new Phase 3 tests), 118 FunctionalParadigm, 92 Utilities (1 pre-existing skip); build clean with TreatWarningsAsErrors
• Reviewed by c-sharp-reviewer: 0 blockers; all actionable findings fixed (sequential href collection restored, var usage, ConfigureAwait(false) on new awaits, DirectoryHelper empty-list guard, factory argument validation)

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