RGA RGB cacheable production v1 - RK3588 board STRONG_PASS

JPU integration checkpoint before RK3588 board validation

Checkpoint before merging upstream safe batch1

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