Adds Repo.Merge(srcBranch, dstBranch, message, user) which resolves each branch tip, finds the common ancestor, runs ThreeWayText.Merge against the union of source and destination filesets, and — if every file merges cleanly — records a merge commit whose primary parent is the dst tip and whose secondary parent is the src tip (plink isprim=1 / 0).

Any unresolved conflict aborts the whole operation with a *MergeConflictError listing the affected files; callers can check for it via errors.Is(err, ErrMergeConflict). Agreed deletions are honored, and one-sided modify/delete conflicts are reported as conflicts rather than silently dropped.

Supporting changes:

- CheckinOpts.MergeParents plumbed through manifest.Checkin so the P-card and plink rows carry every parent (previously the fresh-commit path only tracked one parent, unlike the sync path in crosslink.go). - manifest.Checkin now calls tag.PropagateAll(primaryParent) after inline T-card processing, mirroring crosslink.applyInlineTags. Without this, a child commit on an existing branch would not inherit its parent's branch tag, so BranchTip would return stale tips for any non-initial commit. - Repo.BranchTip(name) resolves a branch to its latest checkin via tagxref; secondary ORDER BY rid keeps the lookup deterministic when multiple commits share a julian-day bucket. - CommitOpts.MergeParents exposed on the public API so arbitrary callers can author merge commits without going through Repo.Merge.

Tests cover clean merges of divergent edits, conflicting edits, one-sided additions, missing branches, same-branch input, multi-parent plink verification, and BranchTip resolution on both trunk and named branches. Green on both the modernc and ncruces drivers plus the OTel observer. check-in: 461f168708 user: [email protected] tags: trunk, v0.2.0