Use by-default memfd and background copy

[bchalios]

Make memfd for guest memory and background copies during pause/snapshot the default. The behaviour is still controlled by LaunchDarkly (when present) but the default now is to use these two features.

Also, flip -use-memfd to -disable-memfd flag in create-build and resume-build, so that local testing uses memfd by default and apply the same flip in CI testing environments.

[chatgpt-codex-connector]

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[cursor]

PR Summary

Medium Risk
Default-on changes affect the core pause/snapshot memory path for every environment that relies on flag fallbacks; LaunchDarkly can still turn features off, and CI keeps one shard on the legacy path.

Overview
Guest memory via memfd and asynchronous memfd copy during pause/snapshot are now the default LaunchDarkly offline fallbacks for use-memfd and memfd-background-copy, so new runs use that path unless flags override it. Local CLI tools and CI were switched from opt-in (-use-memfd, TESTS_USE_MEMFD) to opt-out (-disable-memfd, TESTS_DISABLE_MEMFD); integration matrix values were inverted so the uncompressed shard still runs without memfd while compressed shards exercise the new default.

^{Reviewed by Cursor Bugbot for commit bfe9011. Bugbot is set up for automated code reviews on this repo. Configure here.}

[gemini-code-assist]

Code Review

This pull request enables memfd-backed guest memory and background copying by default. It replaces the previous enablement flags and environment variables (use-memfd, TESTS_USE_MEMFD) with disablement alternatives (disable-memfd, TESTS_DISABLE_MEMFD) across the orchestrator, build tools, and GitHub actions. There are no review comments to address, and I have no additional feedback to provide.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/orchestrator/cmd/create-build/main.go	0.00%	3 Missing ⚠️
packages/orchestrator/cmd/resume-build/main.go	0.00%	3 Missing ⚠️
packages/orchestrator/main.go	0.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[dobrac]

make sure to properly specify the FC version dependency