Add Fast Forward Speed Picker experimental feature

[adamgrushan]

Mark the type contribution you are making:

• Experimental feature (new functionality that can be selectively enabled/disabled)
• Bug fix (non-breaking change which fixes an issue)

Description

Summary of your changes, including:

• Why is this change necessary?

Delta's existing "Custom Fast Forward Speed" experimental feature requires configuring a preferred speed per system (NES, SNES, N64, etc.). Many players just want one fast-forward speed everywhere without setting it seven times. This adds a "Fast Forward Speed Picker" experimental feature: a single picker option (2x, 3x, 4x, 8x, or Maximum) that applies across all systems. Speeds above a system's maximum supported rate are automatically capped at that maximum, so one global choice is always safe.

• Why did you decide on this solution?

• The feature follows the "Picker" option pattern from Docs/ExperimentalFeatures.md, reusing the existing FastForwardSpeed type (already LocalizedOptionValue-conforming) so the picker displays consistent "2x"-style labels and a "Maximum" nil choice.
• The implementation is isolated in one new file (Delta/Experimental Features/Features/FastForwardSpeedPicker.swift) containing the options struct and a GameViewController extension, per the isolation guidance.
• The only change to existing logic is a new lowest-priority branch in performFastForwardAction's speed resolution: per-game speed > per-system speed (Custom Fast Forward Speed) > global picker speed > system maximum. This keeps the new feature composable with the existing one and guarantees no behavior change when disabled (the gating property returns nil unless ExperimentalFeatures.shared.fastForwardSpeedPicker.isEnabled).
• No changes to DeltaCore or any emulator cores.

Testing

List all iOS versions and devices you've tested this change on.

Example Configurations:

• iPhone 16 Pro, iOS 27.0 (beta, build 24A5370h)
• Full workspace build for iOS Simulator (arm64) with zero warnings in the changed files

Checklist

General (All PRs)

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I've tested my changes with different device + OS version configurations (tested on one physical device so far — happy to test additional configurations if needed)

Experimental Feature-specific

• Added property to ExperimentalFeatures struct annotated with @Feature
• Uses @Option's to persist all feature-related data
• Locked all behavior changes behind ExperimentalFeatures.shared.[feature].isEnabled runtime check
• Isolates changes to separate files as much as possible (e.g. via Swift extensions)

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