rfc: branch operations v2

[aaltshuler]

What & why

Branch operations still reconstruct growing manifest history, and ancestry-eligible merges can replay source data into target-owned datasets. Propose RFC 0060 to replace current-state reconstruction with explicit immutable graph snapshots and conditional adoption commits, preserving the target's first-parent history.

The draft defines one current __manifest authority for logical refs, exact retained snapshot roots, independent writes after adoption, graph-wide immutable tags, historical reads/forks, merge preview, and safe logical deletion. Whole-state restore follows; rebase, cherry-pick, and selective revert require a later logical-change replay contract.

This PR changes only the RFC and registry. It remains draft / not-started and does not accept a new format or activate runtime behavior.

Backing issue / RFC

• Is an RFC PR: RFC 0060: Branch operations v2

Related work: accepted RFC 0057 / #638, branch-context and retained-ancestry work in #662, and bounded merge preparation in #668. The RFC records scoped relationships to existing authority, retention, recovery, feed, and GQ decisions. No existing decision is superseded by this draft.

Notes for reviewers

• The candidate uses strict native Lance Overwrite with engine-owned fresh retries. Cold latest-version discovery, central-CAS contention, and retained metadata amplification require qualification.
• Adoption must preserve source isolation, first writes, tags, Blob dependencies, and deletion/cleanup safety. Initial destructive cleanup requires a quiesced graph; no new distributed fencing is claimed.
• Existing-graph cutover must preserve selected contents and complete parent-record closure, or explicitly refuse/reset identities and feed baselines.
• The completed AWS campaign is divergent-merge bottleneck evidence: 50 native-S3 measurements and eight NVMe controls. It does not measure promotion speedup. Small eligible fixtures, full first-use/reclamation lifecycles, and activation gates are specified separately.

Checklist

• One focused RFC and registry update.
• Tests for behavior changes: N/A; documentation only. Existing test owners and future qualification gates are specified.
• Public supported-behavior docs: N/A; no new behavior ships.
• Reviewed against architectural invariants and pinned Lance 11 surfaces; unresolved representation/protocol gates remain explicit.

Local verification

• python3 scripts/check-docs.py — passed (132 Markdown files).
• bash scripts/check-agents-md.sh — passed.
• typos docs/rfcs/0060-branch-ops-v2.md docs/rfcs/README.md — passed.
• git diff --cached --check — passed.
• Independent design and benchmark-evidence review — completed.
• Rust builds, tests, DST, and benchmarks — not rerun for this documentation-only PR.