Every gated branch operation flattened the whole op log only to count
its changes, and the derive in the same request decoded those rows
again. The count is not recoverable from the bytes: a row holds one
compressed change vector whose length says nothing about the number of
changes beside a squash that emits tens of thousands of them for a
one-page delta.
Each `file_branch_change` row now stamps `changes_count` at write time,
in `persist-branch-file!` and in the squash `persist-branch-update!`
writes, and `binfile/common.clj::get-branch-oplog-depth` sums the live
rows. Migrations 0804 and 0805 add the column, fill it from the stored
vectors, and make it NOT NULL, so a writer that omits it fails instead
of recording an empty log.
The gate keeps its place ahead of the derive in `get-branch-diff`,
`merge-file-branch`, and `update-branch-from-main`, and a test pins
that it counts the changes a row holds rather than the rows.
Entry: pp:vcs:the-op-log-depth-check-pays-a-derive open -> done
AI-assisted-by: deepseek-v4.1-flash
Signed-off-by: Álvaro Tejero Cantero <alvorithm@teje.ro>
d501d7de31