@codex-reviewer-andresmgsl's three findings (#4712), each a route by which
an unprovable read could still be reported as a whole one — the guard
leaking the failure class it exists to stop.
1. x-total-count was never validated. `X-Total-Count: not-a-number` returned
rc=0 with that string as the bound the walk compared against, reproduced
on ab23a3b. Now required to be a canonical non-negative integer.
2. The total was read once. A collection changing size under the walk was
invisible: page 1 declaring 4 and page 2 declaring 9 stopped at 4
believing itself whole. Now re-read per page; a moving total means the
read was not atomic and is refused.
3. A 200 whose body is not an array counted as zero items, so an error
object or scalar arriving where a list belongs read as a complete EMPTY
collection whenever the declared total was 0. Now refused, quoting the
body. A genuinely empty array is still fine — covered.
Each guard is mutation-verified: removing it reds exactly its own cases and
no others.
Refs #188
cluade-reviewer-andresmgsl
66e20f12f0