Rewrite IFViewAnimator using CoreAnimation (fixes #46)

[angstsmurf]

Summary

Fixes #46 — the source ⇄ Contents and next/previous-section transitions in the Source pane were choppy.

IFViewAnimator drove its transition from a 100 Hz NSTimer that called -setNeedsDisplay: on every tick, with -drawRect: manually compositing two cached bitmaps on the CPU each frame. Because the timer runs on the main thread and isn't vsync-synced, it is easily starved and the animation stutters.

This rewrites the animator to use CoreAnimation: the "from" and "to" views are snapshotted into two CALayers and slid (or cross-faded) with CABasicAnimation, which is GPU-composited and display-link synced.

Details

• The public API is unchanged, so the callers in IFSourcePage.m need no changes.
• The four slide directions actually used (IFAnimateLeft/Right/Up/Down) are preserved exactly — I traced each one's geometry from the old -drawRect: so the direction of motion matches the previous behaviour.
• The unused IFAnimateCrossFade, IFFloatIn, and IFFloatOut styles fall back to a simple cross-fade so the API keeps working.
• Layer contentsScale is set to the window's backing scale factor, so snapshots stay crisp on Retina displays (the old CPU path did not account for this).
• The old mid-animation "recache" hack in -drawRect: (re-snapshotting the target if it still wanted display in the first 25%) is replaced by forcing layoutSubtreeIfNeeded + displayIfNeeded before taking the snapshot.
• Re-entrancy: finishAnimation tears down the layers and is guarded by the existing originalView != nil check, so the CATransaction completion block is a safe no-op if the animation was abandoned early or the animator was deallocated.

Net change is a single file, +145 / −265 lines.

Testing

Built and run in the Xcode IDE. Verified visually:

• Source ⇄ Contents toggle (both directions).
• Next / previous section navigation, including at the first/last section boundaries.
• First responder is restored to the text view after the transition completes.

Base branch is extensions since that is where current development seems to live.

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