docs: freeze R0 closure gates

[NoctilumeDev]

Closes #9

Summary

• separate stable R0 toolchain rules from dated host evidence
• freeze the minimum cross-layer contract and Linux reference-lab protocol
• add an explicit R0 closure gate and hard prohibition on automatic R1 entry
• keep FlowKernel at Planned with no target implementation

Evidence state

• R0 design freeze only
• executable tool lock, machine fixtures, first Linux evidence run, and clean recovery remain pending

Verification

• node --check scripts/verify-repository.mjs
• node scripts/verify-repository.mjs
• git diff --check