shearwater-cloud: update checksum

[coyaSONG]

The vendor replaced the 2.12.10 DMG; this updates the checksum to match the current official artifact.

Built and tested locally on macOS 26.

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After making any changes to a cask, existing or new, verify:

• The submission is for a stable version or documented exception.
• brew audit --cask --online shearwater-cloud is error-free.
• brew style --fix Casks/s/shearwater-cloud.rb reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused (add your cask's name to the end of the search field).
• brew audit --cask --new shearwater-cloud worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask shearwater-cloud worked successfully.
• brew uninstall --cask shearwater-cloud worked successfully.

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• AI was used to generate or assist with generating this PR. AI assisted with diagnosing the checksum mismatch and preparing the one-line update. I manually verified the official download's SHA-256, DMG integrity, app version, the unchanged zap path, and the audit, style, install, and uninstall results.

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