feat(kernel): add misc rust-null and split userspace/

[1907]

Summary

• Move the crate under userspace/ (workspace root).
• Add C misc module kernel/rust_null.c: misc_register as /dev/rust-null, fops copied from null_fops (read 0, write count, llseek always 0).
• Add RfL MiscDevice source in kernel/rfl/ for a later CONFIG_RUST=y kernel (not built by make -C kernel).
• /dev/null vs /dev/rust-null tests skip when the node is missing.

Scope

• One focused task
• No unrelated refactor or formatting churn

Quality gate

• Tests added/updated
• cargo fmt --check, clippy -D warnings, cargo test --locked passed
• make -C kernel produced rust_null.ko (not loaded)
• README/docs/CHANGELOG updated
• No secrets; public repo surface clean
• Independent review: passed (no security/logic blockers)

Verification

• cargo test --locked: contract 6, against /dev/null 4, compare_devices 6 skip-ok, doctest 1
• echo hello | cargo run -p rust-null --example discard --locked exit 0
• make -C kernel on 6.12.107+deb13-amd64: rust_null.ko vermagic matches; not insmod
• /dev/rust-null absent after the build

Risk and rollback

Distro kernels still cannot compile the .rs. make load is opt-in and was not run. Revert the squash if the layout is wrong.