Function gateEdit

The conservative write gate. Given the (before, after) text of a section or article, decide whether the edit preserved it. On ok: false the caller MUST discard after and keep before — the vault stays unchanged.

Enforces (deterministically, no GPU):

1. Citation preservation — every pre-existing conv:HASH survives.
2. Footnote bijection — every [^N] marker ⇔ one [^N]: definition.
3. Prose preservation — the edited word count stays at/above the floor.
4. Structural survival — an H1 and ## References present before survive.

Catches citation/structure/bulk-prose loss; does NOT catch nuance loss.