Add native zoom transitions to the built-in image viewer

[linzhiyi (LimChihi)]

Summary

The built-in fullscreen image viewer currently opens without a transition from the tapped image. Add native zoom presentation and dismissal on iOS 18 and later, entirely within the library.

• Give each image occurrence its own transition namespace, including repeated resources.
• Let the system handle dismissal at minimum zoom; preserve image panning and prevent interactive dismissal while zoomed or pinching.
• Keep iOS 16/17 presentation behavior. No host-provided viewer, transition source, or new public API is needed.

Closes #185

Validation

• make ci passes: lint, 120 XCTest cases (including the new fullscreen transition test), 9 Swift Testing cases, existing snapshot validation, and sample app build.
• Independent simulator UI harnesses pass the same fast downward dismissal gesture with MarkdownView, StreamedMarkdownView, and a UIKit-hosted Markdown wrapper.
• Consumer-app image journey UI tests pass: 5/5 on the iOS 27 simulator, covering fast/slow downward dismissal, short-drag cancellation, pinch zoom and zoomed panning, repeated image occurrences, content/edge swipes, and close.
• An earlier fast-dismiss integration run failed; the original test input subsequently passed in both a targeted run and the complete suite. The cause of that earlier failure is unconfirmed. No physical-device verification.

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed. (new transition test fixture; README unchanged)
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed. (no dependency changes)
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

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   PROVIDED WITHOUT WARRANTY OF ANY KIND, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTY OF
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    supersedes any and all prior agreements, understandings or communications, written or oral, between
    the parties relating to the subject matter hereof. This Agreement may be assigned by Microsoft.

[Copilot]

🟡 Changes recommended

The test bypasses production transition-source wiring and does not protect repeated-image behavior.

1 open finding

• Test transition source wiring for repeated rendered Markdown resources · New

What changed in this PR

Adds iOS 18 native zoom transitions to the built-in fullscreen image viewer while preserving earlier iOS behavior.

Changes:

• Adds per-image transition namespaces and native zoom presentation.
• Coordinates panning, pinching, and interactive dismissal.
• Adds accessibility identifiers and transition coverage.

File	Description
BlockImageView.swift	Defines each inline image as a unique transition source.
ImageViewerView.swift	Applies the native destination transition and fullscreen layout.
PinchZoomView.swift	Coordinates custom gestures with native dismissal.
ImageViewerTransitionTests.swift	Tests presentation transition configuration.

🧠 Review effort: Balanced

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