[wip] use opportunistic reflinks for copying files

[thisguycodes]

Description

#5608 should go in first, this depends on it, and I want to get test timings from it.

Adds opportunistic reflink usage (clonefile and ioctlficlone) when copying files, particularly in tests. Reflinks result in no data needing to be copied / moved (besides a new inode created).

TODOs

Read the Gruntwork contribution guidelines.

• I authored this code entirely myself
• I am submitting code based on open source software (e.g. MIT, MPL-2.0, Apache)]
• I am adding or upgrading a dependency or adapted code and confirm it has a compatible open source license
• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Use opportunistic reflinks when copying files

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Project	Deployment	Actions	Updated (UTC)
terragrunt-docs	Ready	Preview, Comment	Mar 31, 2026 4:09pm

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