Checkpoint: #1534 cow.rs mmap EOF fix verified (fmt+clippy+QEMU smoke pass) Base: fdc633268 (master) Commit: e6b6218e1 Upstream: PR #1534 (commit ddbd8bfed) File: os/StarryOS/kernel/src/mm/aspace/backend/cow.rs (+61 -3) Validation: - cargo fmt --check: PASS - cargo xtask clippy --package starry-kernel: 0 new errors (51 pre-existing baseline) - x86_64 QEMU smoke: boot to shell, rootfs mount OK, no cow panic