spec: migrate RFCs to spec/rfcs/ with template + lifecycle doc (closes #66)

[proofmancer]

Closes #66.

Promotes the five open RFCs from "GitHub issues with the design label" to structured markdown files with frontmatter, in a dedicated directory with a documented lifecycle. Standard pattern for any language project that wants to be open to community contributions; this matches what Rust, Ethereum, Cosmos, and Bitcoin all do.

What landed

Directory + process docs

• `spec/rfcs/README.md`: explains what gets an RFC, how to open one, the lifecycle (draft / accepted / implemented / superseded / rejected), the frontmatter format, and what an RFC is not
• `spec/rfcs/0000-template.md`: copy-this-for-new-RFCs template with sections for summary, motivation, design, alternatives, drawbacks, open questions, reversibility, related work, implementation roadmap

Migrated RFCs

Five proposals lifted from issue bodies into structured files:

File	Tracking
`spec/rfcs/0001-memory-model.md`	#42
`spec/rfcs/0002-extern-host-abi.md`	#55
`spec/rfcs/0003-project-metadata.md`	#45
`spec/rfcs/0004-cross-engine-state.md`	#46
`spec/rfcs/0005-third-party-protocols.md`	#65

Each gets YAML frontmatter:

```yaml

rfc: 0001
title: "Memory model for Cleave (ownership, GC, escape hatches)"
status: draft
authors: ["Cleave Labs"]
tracking: #42
created: 2026-05-23

```

The body text is unchanged from the issue. Substantive edits happen via PRs against the file going forward.

Cross-linking

Each tracking issue gets a comment pointing at its new RFC file. Discussion continues on the issue; the file is the source of truth for proposal text.

Spec index

`spec/README.md` table updated:

• Adds `spec/rfcs/` entry pointing at the RFC index
• Marks module bodies, type system, ABI as stable now (was planned); they shipped in v0.3
• Marks the standard-library reference as planned (v0.5+) (was planned (v0.2))

Why now

Five RFCs is the inflection point. Below that, GitHub issues are fine. Above, structure matters:

• A markdown file has a stable URL anyone can cite
• Frontmatter gives at-a-glance state
• A template makes RFCs consistent and easier to write
• The process docs onboard external contributors
• Numbering establishes a citation surface

What this PR does NOT do

• No automation: no `make rfc` script, no lifecycle-state bot, no RFC-to-implementation linker. Manual for now; volume does not justify tooling.
• No new RFCs: every RFC in this PR existed before, just as issue text. Six RFCs (0000 template + 0001-0005) is the new baseline; the next one to be filed is 0006.
• Does not move `spec/protocols/*.md`: those are stable specs for shipped interfaces (consensus, gas, state, da, effects RFCs). RFCs and stable specs are different artifacts: an RFC is a proposal; a spec is the stable contract once accepted and implemented. Future RFCs about protocols start in `spec/rfcs/`, graduate to `spec/protocols/` once shipped.

What unblocks next

• External contributors can now file RFCs by opening a PR against the template
• The position paper (Paper: design sketch / position paper for arxiv (defer active drafting) #67) can cite specific RFC numbers
• Future implementation work links to the RFC by number + commit hash rather than to issue text that might drift