Untrack the giant snapfile (2.4GB)

[buddhisthead]

This PR does the minimal fix needed so that when people clone the repo, they don't suffer a giant 2.4GB download of the snapshot ledger file. I mistakenly added this large snapshot file when I should have created it as a release asset or just had a makefile target to download the file.

The alternatives are:

1. rewrite history after removing the file. This is a blunt weapon that is a bad idea. It wrecks existing PRs.
2. remove the LFS tracking from the file on HEAD, so a fresh clone will not download the file. This is clean. Old branches may pull the file, but I don't think anyone will experience that. Updates the Makefile to download the snapshot file when needed for testing. I'll update again to support booting from snapshot, but that will also be on-demand and not in the repo.

Option 1 is a bit dangerous and I'll leave that for the future if we strongly need it. So I'll go with option 2 here.

I've also added a .gitignore entry so that the file can't accidentally be committed in the future.

[Copilot]

Pull Request Overview

This PR removes Git LFS tracking for a 2.4GB test fixture file to prevent it from being downloaded during fresh repository clones. The change removes both the LFS pointer file and its .gitattributes entry, allowing the repository to be cloned without the large file burden while preserving it in history for older branches.

Key Changes:

• Removed Git LFS pointer file for the large CBOR test fixture
• Removed .gitattributes entry that configured LFS tracking for this file

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/fixtures/134092758.670ca68c3de580f8469677754a725e86ca72a7be381d3108569f0704a5fca327.cbor	Removed LFS pointer file to stop tracking the 2.4GB fixture
tests/fixtures/.gitattributes	Removed LFS configuration entry for the fixture file

Comments suppressed due to low confidence (1)

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