Order the Docs batch back to front, not merely reversed

[ptheofan]

Pushing a markdown file that differs wholesale from the Doc failed with
Invalid deletion range. Cannot delete the requested range. The batch
began with a ~1,900-character insertText at index 1, followed by three
deletes at descending indices.

Root cause. Google applies a batch sequentially — each request sees
the document as the previous ones left it. diffOpsToRequests ordered
the batch by reversing the generated list, which is not the same as
ordering it back-to-front: operations are generated in document order and
an N:M replacement pushes its deletes before its insert, so the reversal
put the insert first, at the same index those deletes start from. Every
delete after it then landed ~1,900 characters late.

Fix. Sort explicitly — descending by index, deletes before inserts at
the same index, so a position is cleared before anything is written to it.

Verification

The new test applies the emitted batch the way Google does, sequentially,
and refuses an out-of-range delete as the API would. Against the shape of
the failing document — three paragraph runs split by two tables, replaced
wholesale — it failed with:

expected '\nAAAA\nBBBB\nCCCC\nDDDD\n' to be 'ZZZZ\nYYYY\n'

Every original paragraph had survived and the new content was destroyed.
It passes after the sort, with both tables untouched.

The existing "reverse order for index stability" test passed throughout —
it asserts indices are non-increasing, which an insert tied with a delete
satisfies. That is why this survived: the ordering was checked, the
outcome of applying it was not.

Known artefact, not fixed here. A paragraph delete stops one
character short of its run, so each replaced run leaves a trailing
newline behind as an empty paragraph. The short stop is deliberate —
taking the newline before a table is itself invalid — and removing the
leftovers needs the delete to know whether a table follows. Filed in the
issue rather than bundled in.

pnpm test 1034 passed / 62 files · typecheck clean · lint clean.

Closes #54

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