Add FUSE remap_file_range support for FICLONE/FICLONERANGE

[ejc3]

Summary

Enables cp --reflink=always and FICLONE/FICLONERANGE ioctls through FUSE by adding remap_file_range support to the full stack.

Problem: FICLONE fails with EOPNOTSUPP through FUSE because there's no kernel opcode for it.

Solution:

• Kernel patch adds FUSE_REMAP_FILE_RANGE opcode (54)
• fuse-pipe protocol extension for RemapFileRange
• Passthrough implementation using copy_file_range (which handles FICLONE on btrfs)
• Integration tests that skip gracefully on unpatched kernels

Full Stack Implementation

Layer	Component	Changes
Kernel	kernel/build.sh	Inlines FUSE_REMAP_FILE_RANGE patch (opcode 54, struct, handler)
fuser	ejc3/fuser:remap-file-range	Opcode constant + remap_file_range trait method
fuse-backend-rs	ejc3/fuse-backend-rs:remap-file-range	PassthroughFs impl using copy_file_range
libfuse	ejc3/libfuse:remap-file-range	passthrough_ll.c impl for C-based testing
fuse-pipe	src/protocol/, src/client/, src/server/	Wire protocol + handler
Tests	tests/test_remap_file_range.rs	VM end-to-end test
Tests	fuse-pipe/tests/test_remap_file_range.rs	Host-side FUSE tests
Tests	Containerfile.libfuse-remap	libfuse container test

Key Fixes During Development

1. Opcode 54 (not 53) - opcode 53 is reserved for FUSE_SYNCFS
2. len=0 semantics - FICLONE passes len=0 meaning "to end of file", fixed underflow in writeback range calculation
3. Return value - must return bytes cloned via fuse_write_out, not just success/fail
4. Exit code handling - cp --reflink=always returns 1 (not errno) on failure

Test Behavior

Tests skip gracefully without patched kernel - won't break CI:

Test	Skip Condition
test_ficlone_cp_reflink_in_vm	No btrfs, or exit code 1/38/95
test_ficlone_whole_file	ENOSYS or EOPNOTSUPP from kernel
test_ficlonerange_partial	ENOSYS or EOPNOTSUPP from kernel

To run with patched kernel:

REMAP_KERNEL=/mnt/fcvm-btrfs/kernels/vmlinux-6.12.10-*.bin make test-root FILTER=remap

Verified Working

remap_file_range called ino_in=2 fh_in=2 offset_in=0 ino_out=3 fh_out=3 len=0 remap_flags=0
remap_file_range response response=Written { size: 0 }
[ctr:stdout] FICLONE test passed
✓ Verified: files share physical extents (true reflink)
[REMAP-VM] ✓ ficlone (6.9s)

Test plan

• cargo build --release passes
• make test-root passes (tests skip on standard kernel)
• REMAP_KERNEL=... make test-root FILTER=remap passes with patched kernel
• libfuse container test passes: FICLONE + FICLONERANGE (offset=0 and offset=512KB)
• filefrag confirms shared extents (true reflinks on btrfs)