test262: add --max-gaps gate, wire CI at 0 to lock in zero engine gaps

A --max-gaps=<n> harness flag (mirrors --min-pass-pct) exits 2 when the
engine-gaps count — failures explained by neither a policy class nor the
body-audit registry — exceeds <n>. Unlike the pass% floor it is NOT
filter-exempt: a gap count is meaningful at any granularity, so
`--filter=<area> --max-gaps=0` gates just that area's gaps.

The gating test262 CI job now passes --max-gaps=0, so a new unaudited gap — a
real engine bug, or a by-design fixture from a test262 submodule bump not yet
registered in tools/test262/gap_audit.zig — fails the build until it is triaged
(--list-gaps prints the list). Turns the current 0-engine-gaps state into an
enforced invariant instead of a number nobody diffs.

Verified: exit 2 when gaps exceed the budget, exit 0 under it, and 0 gaps today
passes --max-gaps=0.