docs: define C-first kernel charter

[NoctilumeDev]

Summary

• define FlowKernel as a planned C-first experimental kernel rather than a Linux-hosted final design
• formalize the freestanding C trusted core, minimal assembly boundary, explicit lifecycle state machine, deterministic Guard, and bounded learning-policy role
• keep Linux, cgroup, sched_ext, and eBPF as a reference lab and control environment
• align the README, vision, hypotheses, roadmap, research questions, threat model, and evidence policy

Validation

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

Evidence boundary

This remains a Planned research repository. No runnable kernel or validated performance claim is introduced by this PR.