Fix default FXL spread positions

[mickael-menu]

Fixed

Navigator

• The first resource of a fixed-layout EPUB is now displayed on its own by default, matching Apple Books behavior.
• Fixed the default spread position for single fixed-layout EPUB spreads that are not the first page.

[Copilot]

Pull request overview

Adjusts fixed-layout EPUB spread construction and JSON page positioning so the first FXL resource is displayed alone by default (Apple Books-like), and unpaired single pages get a corrected default left/right position.

Changes:

• Updates FXL two-page spread generation to default the first resource to a centered single spread (unless overridden by offsetFirstPage or explicit page property).
• Reworks default page position for EPUBSingleSpread JSON output to distinguish first vs. non-first single pages.
• Adds comprehensive navigator spread unit tests and documents the fixes in the changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
Tests/NavigatorTests/EPUB/EPUBSpreadTests.swift	Adds new unit tests covering spread creation, pairing rules, and spread helper APIs.
Sources/Navigator/Preferences/Types.swift	Removes ReadingProgression.startingPage helper used for default positioning.
Sources/Navigator/EPUB/EPUBSpread.swift	Implements new first-page centering behavior for FXL and fixes default page position for single spreads.
CHANGELOG.md	Records the Navigator fixes under Unreleased.

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[stevenzeck]

Can all be parameters be documented here? swift build -Xswiftc otherwise throws warnings for parameters that aren't documented if at least one is.

[mickael-menu]

I updated to remove the - Parameter and inline the explanation for offsetFirstPage instead. I'm not a fan of having a list of - Parameter when they are self-explanatory, but I would compromise for public APIs.

Are you able to only document the public APIs? I think documenting (or checking) all the internal APIs would add too much noise.

[stevenzeck]

Since this is new, it would be helpful to use the returns section. https://developer.apple.com/library/archive/documentation/Xcode/Reference/xcode_markup_formatting_ref/Returns.html

[mickael-menu]

I don't think this improves the Quick Help, as this is the short description of what this function does. I prefer to use - Returns to clarify the return value in the context of the function. For example:

swift-toolkit/Sources/Navigator/Input/InputObservable.swift

Lines 12 to 17 in bc32270

	/// Registers a new ``InputObserver`` for the observable receiver.
	///
	/// - Returns: An opaque token which can be used to remove the observer with
	/// `removeInputObserver`.
	@discardableResult
	func addObserver(_ observer: InputObserving) -> InputObservableToken

Before:

After:

[mickael-menu]

In this case, I think I should have used "Resolve the default spread position..." instead of "Returns...". That was lazy wording 😅