Address three needs-judgment items from the cursor-review judge synthesis:
1. Cursor wire format now includes an "o" key carrying the sort
direction ("asc" / "desc") it was minted under. A request that
replays the cursor with a flipped `order` parameter is rejected
with 400 INVALID_CURSOR instead of silently walking the wrong
direction. Legacy cursors without "o" still decode (the binding
is best-effort until cloud mirrors the field — follow-up filed
separately).
2. JSON serialization now escapes `<`, `>`, `&`, U+2028, U+2029
to mirror Go's default `json.Marshal` behavior. Without this, an
asset name containing those characters produced different bytes on
Python vs cloud Go. The escaped form is what both runtimes emit.
3. Add direct query-layer tests for the keyset tiebreaker — the secondary
ORDER BY id branch was previously unexercised. Two scenarios: all
rows share a primary sort value, and mixed ties straddle page
boundaries. Both assert no row is dropped or duplicated across the
walk.
Wire-format note: Python cursors now differ from current cloud cursors
by exactly the "o" key. Cloud follow-up will bring the two back into
byte alignment.
