rfc: add engine version 2 placeholder

[azimafroozeh]

What & why

This PR adds RFC 0067, a placeholder that names engine version 2 as a list of components so that each component's own RFC has one parent to cite.

• The planner RFC is the first component RFC in preparation; without a parent, every component RFC would restate the whole v2 scope.
• The table lists only the components no accepted RFC owns: planner and optimizer, execution engine, memory management and admission control. Components an accepted RFC already owns (write protocol, conflict fencing, recovery, schema identity) are not repeated.
• Registry: row added, next available number moved from 0066 to 0068, with RFC 0066: Server lifecycle and online deployment #697's 0066 reservation recorded.

Backing issue / RFC

• Is an RFC PR: docs/rfcs/0067-engine-version-2.md

Checklist

• Change is focused (one new RFC document plus its registry row)
• Tests added/updated for behavior changes (N/A: documentation only, no behavior changes)
• Public docs updated if user-facing surface changed (N/A: no user-facing surface; maintainer-track RFC)
• Reviewed against docs/dev/invariants.md — no Hard Invariant weakened, no deny-list item hit (no invariant touched; the RFC's Invariants section states none affected)

Local verification

• python3 scripts/check-docs.py — OK, 139 files
• Template section order and em-dash check with a local lint script — 0 findings
• cargo test --workspace — not run: no Rust files changed

Notes for reviewers

• Number 0067 follows RFC 0066: Server lifecycle and online deployment #697's reservation of 0066; the provisional-number note under the title stays until the number is confirmed at merge.
• Merges as draft; implementation: not-started tracks the component table only.
• Adding or removing a component row is a Decision-log entry in this RFC, not a new document.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-14T15:50:24.417841Z	463fae4	PR opened

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💡 Codex Review

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Reviewed commit: 463fae4e02

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Track delivered code in the implementation field

When a component RFC lands before its code, this instruction makes RFC 0067's implementation status advance merely because a table cell changed, so the canonical registry can report engine-v2 implementation progress—or even completion—while every component remains unimplemented. Keep this parent at n/a if it is only an index, or derive its implementation state from shipped component work rather than proposal bookkeeping.

AGENTS.md reference: AGENTS.md:L3-L5

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