Improve and cleanup cask downloads

[MikeMcQuaid]

• Install.enqueue_cask_installers drained cask downloads under a separate Downloading Cask files heading before the shared queue fetched formula bottles, serialising the two and splitting the output.
• Leave cask and dependency downloads queued so every command fetches them concurrently with bottles under one Fetching downloads for: heading, marking failed cask downloads afterwards.
• Drop the cask source file fallback: language variations and structured install steps live in the API, so caskfile_only? and Homebrew::API::Cask.source_download* never fired for official casks.
• Stop publishing api/cask-source/*.rb and treat cached cask sources as stale so existing caches are cleaned up.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Fable 5 high with local review and testing.

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[Copilot]

Pull request overview

This PR consolidates cask and formula downloads, removes obsolete cask source fallbacks, and cleans stale cached sources.

Changes:

• Queues cask and dependency downloads with formula downloads.
• Removes cask source API fallback and publication.
• Updates cleanup behavior and related tests.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Summary
Library/Homebrew/test/install_spec.rb	Tests cask enqueueing and failure handling.
Library/Homebrew/test/cmd/upgrade_spec.rb	Updates shared-fetch upgrade expectations.
Library/Homebrew/test/cmd/install_spec.rb	Updates shared download expectations.
Library/Homebrew/test/cleanup_spec.rb	Tests stale cask source cleanup.
Library/Homebrew/test/cask/upgrade_spec.rb	Updates cask upgrade tests.
Library/Homebrew/test/cask/reinstall_spec.rb	Updates cask reinstall tests.
Library/Homebrew/test/cask/installer_spec.rb	Removes source fallback tests.
Library/Homebrew/test/cask/cask_spec.rb	Removes obsolete source expectations.
Library/Homebrew/test/cask/cask_loader/from_api_loader_spec.rb	Removes legacy fallback assertions.
Library/Homebrew/test/api/cask_spec.rb	Removes source download API tests.
Library/Homebrew/install.rb	Critical (1 vote): Dependency resolution may duplicate an asynchronously queued cask download and race on cache writes.
Library/Homebrew/dev-cmd/generate-cask-api.rb	Moderate (2 votes): Existing generated cask-source files are not removed during regeneration.
Library/Homebrew/cmd/upgrade.rb	Moderate (2 votes): Enqueue failures are reported as success, potentially reusing failed prefetched installers.
Library/Homebrew/cmd/reinstall.rb	Uses shared cask download fetching.
Library/Homebrew/cmd/install.rb	Fetches casks with formula downloads.
Library/Homebrew/cleanup.rb	Treats cached cask sources as stale.
Library/Homebrew/cask/upgrade.rb	Updates cask upgrade prefetching.
Library/Homebrew/cask/reinstall.rb	Updates cask reinstall prefetching.
Library/Homebrew/cask/installer.rb	Removes source fallback and updates dependency handling.
Library/Homebrew/cask/cask.rb	Removes legacy cask-file-only state.
Library/Homebrew/cask/cask_loader.rb	Removes cached-source tap resolution.
Library/Homebrew/api/cask.rb	Removes cask source download APIs.

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