[ShryukGrandhi]

Summary

registerSidebarView is deprecated, but its contract still declares files as a mutable ExtensionDiffFile[]. Registration forwarded the legacy component to registerPane with a double cast, so at render time it received the pane props, including the host's frozen files list. A legacy sidebar that followed its declared contract and called files.sort() or files.push() threw TypeError: Attempted to assign to readonly property and was replaced by the pane error boundary.

Changes

• Add adaptLegacySidebarComponent in packages/hunk/src/extensions/panes.ts. It maps ExtensionPaneProps to ExtensionSidebarViewProps, passes a shallow copy of files on each render, and leaves out the pane-only fields (review, reviewGeneration, placement, height, currentLine). The file views themselves stay frozen.
• registerSidebarView uses the adapter instead of view.component as unknown as ExtensionPane["component"]. It now checks that component is a function before wrapping it, because the wrapper would otherwise get past registerPane's component check.
• Modern panes registered with registerPane are unchanged and still receive the frozen, readonly host list.
• Add a patch changeset.

Testing

• New render-level test in AppHost.extension-sidebar.test.tsx: a legacy sidebar sorts and pushes onto its files, and a modern pane is mounted next to it. The legacy view renders its mutated copy (LEGACY 3 beta.txt), and the modern pane still sees the original frozen list (MODERN 2 alpha.txt frozen=true). Before the fix this test fails with Attempted to assign to readonly property.
• New unit test in runExtension.test.ts for the prop mapping and the copy semantics. Existing registerSidebarView tests now check delegation instead of function identity.
• bun test --timeout 30000 packages/hunk/src/ui/AppHost.extension-sidebar.test.tsx: 16 pass
• bun test packages/hunk/src/extensions/runExtension.test.ts: 52 pass
• bun run typecheck, bun run deps:check, oxlint --deny-warnings, and oxfmt --check on the touched files: clean
• Running all of packages/hunk/src/extensions still shows failures in hostRuntimeModules.test.ts and the extension package-install tests. These fail the same way on unmodified main on this machine (macOS arm64), so this change doesn't cause them.

Fixes #764

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