The specific failure cases this fixes:

1. Duplicate values in edit history — If a user types "hello", changes to "world", then changes back to "hello", the old .reverse().find(e => e.value
   === cell.value) would match the wrong edit (and its wrong validatedBy, wrong author, wrong timestamp). Now activeEditId points directly to the
   correct edit.
2. Validation targeting the wrong edit — validateCellContent() used to scan backward for a value match. With duplicate values this could validate the
   wrong edit entry. Now it looks up by activeEditId first.
3. Merge dedup correctness — During merges, ensureEditHistoryIds() stamps IDs on pre-migration data before dedup runs, so both branches produce
   consistent identifiers for the same edit.
4. Webview isCurrentVersion highlighting — The edit history modal used the same fragile value-matching to highlight the "current" version. Now it
   uses entry.id === props.activeEditId.

What's still fragile (by design): Every ID lookup has a value-matching fallback for files that haven't been migrated yet or edge cases where
activeEditId is missing. Once the migration has run on a project, those fallbacks become dead code paths in practice.