#13328 Prefetch iOS artifact tab content

Problem

Opening several Mac-hosted artifact tabs or paging through terminal artifacts makes each file wait for its own transfer.

Change

• Warm inactive workspace file and Markdown tabs with bounded concurrency.
• Warm previous and next terminal gallery artifacts when the selected page changes.
• Add a shared memory and disk content cache with in-flight transfer deduplication.
• Namespace cached bytes by the active Mac connection generation so reconnects cannot replay stale content.
• Keep prefetch best effort, capped at the existing 64 MiB inline preview limit, with metadata, cancellation, and cache-write guards.

Validation

• swift build --package-path Packages/iOS/CmuxAgentChatUI
• Focused ChatArtifactLoaderTests and ChatArtifactContentCacheTests: 17 passed.
• git diff --check

The complete test target is currently blocked by the existing ChatArtifactViewerErrorStateTests.swift call to the private/static ChatArtifactViewerModel.state helper. The full MobileShellUI package build is unavailable in this checkout because the local GhosttyKit.xcframework has no binary artifact.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}

---

Summary by cubic

Warms Mac-hosted artifact tabs ahead of opening so switching tabs or paging through terminal artifacts no longer waits for a fresh transfer.

• Prefetches inactive workspace file and Markdown tabs with bounded concurrency, plus adjacent terminal gallery artifacts as the selected page changes.
• Adds a shared memory and disk content cache that deduplicates in-flight transfers, so a viewer joining an active prefetch reuses the existing source stream.
• Namespaces cached bytes by the active Mac connection generation so a reconnect cannot replay stale content.
• Large Markdown panels that exceed the rendered-size cutoff now render as a native text view instead of a WebKit surface, keyed by UTF-8 byte count.
• Prefetch is best effort, respects the 64 MiB preview limit, and cancels stale work when the selection changes.
• Stops reload.sh from excluding Info.plist and gives the notification extension a distinct bundle identifier inherited from the host app so it survives device and simulator builds.

Summary by CodeRabbit

• New Features

  • Artifact previews now preload nearby gallery items and inactive panel files.
  • Added best-effort artifact prefetching with file-size limits.
  • Cache entries are isolated across connection generations.
  • Large Markdown content now falls back to readable plain-text display.
  • Markdown documents now use stable identities to refresh correctly when content changes.

• Bug Fixes

  • Concurrent requests for the same artifact share a single transfer.
  • Improved handling when cached artifact persistence is unavailable, with clear errors when required.

• Tests

  • Added coverage for transfers, cache warming, size limits, metadata validation, persistence failures, and Markdown rendering limits.