assert_root_only_membership and its _in_tx twin have silently diverged on the collection-type exemption #2977

Problem

No current user-facing or functional impact exists (two independent DB triggers make the divergent branch unreachable today), but it is a real, git-history-confirmed maintenance hazard: the last person to touch this code already believed — incorrectly, per their own commit message — that they'd fixed both twins.

Root cause

assert_root_only_membership (packages/core/src/db/sqlite_store/relationships.rs:478) reads has_parent != 0 && node_type != "person"; assert_root_only_membership_in_tx (line 528) reads has_parent != 0 && node_type != "collection" && node_type != "person" — an extra clause the non-tx sibling lacks. Commit be5a7d270 ("Enforce that a collection is always a root") deliberately removed the collection exemption from the non-tx function, with a commit message stating explicitly "the collection exemption in both root-only membership guards is gone" — but the diff never touched the _in_tx twin, which still has it. Two DB triggers (collection_is_root_edge, collection_is_root_type) currently make has_parent structurally always 0 for a collection, so the divergence is inert today — but it's a proven authoring hazard, not a hypothetical one.

Suggested fix

Remove the leftover node_type != "collection" clause from assert_root_only_membership_in_tx to match the non-tx twin, and add a shared macro/helper so this pair (and other _in_tx twins in this codebase) can't silently diverge again.

Found via the 2026-09-25 full-repo code sweep (finding #31 of 84 in CODE_SWEEP_FINDINGS_2026-09-25.md). Severity: medium. Category: duplicate.