Make Streamable and Resource protocols Sendable

[stevenzeck]

1. Conforms Streamable and Resource protocols to Sendable across the codebase.
2. Updates the stream callback signatures to require @escaping @Sendable (Data) -> Void.
3. Refactors TransformingResource from subclass inheritance to closure-based composition.
4. Converts BufferingResource, TailCachingResource, and Streamable local accumulators to use Mutex wrappers to prevent data races.
5. Conforms PositionsService to Sendable & AnyObject to allow safe [weak self] closure capture.
6. Resolves strict concurrency warnings in LCPDecryptor and EPUBNavigatorViewModel by pre-serving fonts on the main actor and using read-only snapshots in background mapping tasks.
7. In CBCLCPResource, the size calculation for plainTextSizeTask has been shifted from a lazy var to a constant Task initialized in init to conform to Sendable.
8. Because WebViewServer is main-actor isolated, font URLs are now pre-served on the @MainActor before mapping. A warning is logged if an unserved font is encountered during mapping.

Public API Breaking Changes

1. Streamable, Resource, HTMLFontFamilyDeclaration, and PositionsService all conform to Sendable.
2. The consume callback added @Sendable.
3. Resource.map and Resource.mapAsString transform closures both added @Sendable
4. TransformingResource changed from open class to public final.
5. fontFiles is a required property in HTMLFontFamilyDeclaration.
6. In PositionsService, default implementations for links and get are now restricted to where Self: AnyObject to support safe weak capture. Any custom struct-based conformances must implement these properties manually or be changed to classes/actors.

Note: This will have warnings until #769 and/or #772 are merged.

Relates to #758.

[Copilot]

Pull request overview

This PR moves core resource streaming APIs toward Swift strict-concurrency compatibility by making streaming/resource protocols and related callbacks Sendable, and by refactoring several resource wrappers to avoid shared mutable captures.

Changes:

• Adds Sendable requirements to Streamable, Resource-related callbacks, PositionsService, and HTML font declarations.
• Reworks resource transformations and buffering/tail caching to use closure composition and Mutex-protected accumulators.
• Updates LCP decryption and EPUB CSS/font serving paths to reduce concurrency diagnostics.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
Tests/SharedTests/Publication/Services/Positions/PositionsServiceTests.swift	Converts the test positions service from struct to class.
Sources/Shared/Toolkit/ZIP/ZIPFoundation/ZIPFoundationContainer.swift	Updates ZIPFoundation resource streaming callback to @Sendable.
Sources/Shared/Toolkit/HTTP/HTTPResource.swift	Updates HTTP resource streaming callback to @Sendable.
Sources/Shared/Toolkit/File/FileResource.swift	Updates file resource streaming callback to @Sendable.
Sources/Shared/Toolkit/Data/Streamable.swift	Makes Streamable Sendable and protects local read accumulation with Mutex.
Sources/Shared/Toolkit/Data/Resource/TransformingResource.swift	Refactors transforming resources from subclassing to closure-based composition.
Sources/Shared/Toolkit/Data/Resource/TailCachingResource.swift	Updates streaming callback and protects tail accumulation with Mutex.
Sources/Shared/Toolkit/Data/Resource/FailureResource.swift	Updates failure resource streaming callback to @Sendable.
Sources/Shared/Toolkit/Data/Resource/DataResource.swift	Makes data factories and streaming callbacks @Sendable.
Sources/Shared/Toolkit/Data/Resource/CachingResource.swift	Updates cached resource streaming callback to @Sendable.
Sources/Shared/Toolkit/Data/Resource/BufferingResource.swift	Updates streaming callback and protects buffered accumulation with Mutex.
Sources/Shared/Toolkit/Data/Resource/BorrowedResource.swift	Converts borrowed resource to a struct and updates streaming callback.
Sources/Shared/Publication/Services/Positions/PositionsService.swift	Makes positions services Sendable and wraps positions resource access in a sendable closure.
Sources/Shared/Publication/Services/Cover/GeneratedCoverService.swift	Wraps cover resource access in a sendable weak-self closure.
Sources/Navigator/EPUB/EPUBNavigatorViewModel.swift	Pre-serves fonts on the main actor and snapshots font mappings for CSS injection.
Sources/Navigator/EPUB/CSS/HTMLFontFamilyDeclaration.swift	Makes font declarations Sendable and exposes associated font files.
Sources/LCP/Content Protection/LCPDecryptor.swift	Replaces FullLCPResource subclassing with TransformingResource composition and eagerly creates CBC plaintext size task.

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[stevenzeck]

@mickael-menu I'd prefer not to take this PR too far. Making LCPLicense conform to Sendable will lead to be more warnings, and fixing them leads to a few more. More refactoring can be done through other PRs. Thoughts?

[mickael-menu]

@stevenzeck Agreed, if it's not worse than before and only strict concurrency warnings.

[mickael-menu]

Thank you for making the changes @stevenzeck!