Add reflink/COW support for shared/copy files#21
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Large directories under shared/copy/ (vendor/, node_modules/, .venv/, target/) previously took minutes to duplicate into each worktree via io.Copy. On filesystems that support copy-on-write cloning -- APFS on macOS, btrfs on Linux, XFS with reflink=1 -- the new fscopy package uses clonefile(2) / FICLONE so copies are near-instant and share on-disk blocks until one side is modified. On ext4, NFS, tmpfs, Windows, and across volumes, fscopy falls back transparently to the previous byte-for-byte copy. No configuration or user-visible change required.
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Summary
internal/project/fscopypackage:CopyFiletries reflink first viaunix.Clonefile(darwin) orunix.IoctlSetInt+FICLONE(linux), falls back to byte-copy on Windows/ext4/cross-volume/etc.<dst>.wtclonetmp + rename; interrupted runs self-heal on next invocation.internal/project/apply.go:ApplyCopyto usefscopy.CopyFile; deletes the privatecopyFile.golang.org/x/sysfrom indirect to direct dep (no new transitive deps).Closes bd-3is.
Test plan
make dev(fmt + vet + test + build) green on darwinGOOS=linuxandGOOS=windowsinternal/project/fscopytests cover happy path, empty file, overwrite, stale-tmp cleanup, missing source, missing parent, and thebyteCopyfallback directlyinternal/project/apply_test.gotests continue to pass unchangedshared/copy/->wt addGo process used ~60 ms CPU; byte-equal shasums