Start bottle downloads before dependency fetches

[MikeMcQuaid]

• Start the requested bottle after prelude checks but before recursive dependency enqueueing so network I/O overlaps Ruby work.
• Reuse the cached-location future in DownloadQueue and defer staging until dependency enqueueing completes, avoiding duplicate transfers and early extraction.
• Preserve direct fetch ordering and existing attestation, source, local bottle and only-dependencies paths.
• This favours cold parallel downloads; warm or serial downloads gain little and later dependency failures can waste bandwidth.

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[Copilot]

Pull request overview

This PR adjusts formula fetching to begin bottle downloads earlier (after prelude checks) so network I/O can overlap with Ruby dependency work, while deferring staging/extraction until dependency enqueueing finishes to avoid duplicate transfers and premature extraction.

Changes:

• Start bottle download enqueueing immediately after prelude (when applicable) and before recursively enqueueing dependency fetches.
• Re-enqueue the same bottle after dependency enqueueing to trigger the normal queued behavior (including staging when using a DownloadQueue configured with pour: true).
• Add/extend RSpec coverage for both the new enqueue ordering in FormulaInstaller#enqueue_fetch and for “in-flight download promotion” behavior in Homebrew::DownloadQueue.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/formula_installer.rb	Enqueues bottle downloads earlier (post-prelude) and defers staged handling until after dependency enqueueing.
Library/Homebrew/test/formula_installer_spec.rb	Adds specs asserting the new enqueue_fetch ordering with and without prelude having run.
Library/Homebrew/test/download_queue_spec.rb	Adds a spec asserting that an already-enqueued download can be “promoted” to staged behavior without a duplicate fetch.

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