deps: pin zat at the v0.3.8 tag
Move off the interim commit-archive pin onto the released zat v0.3.8 tag
(same code, now a clean versioned pin). Still on canonical websocket v0.1.8.
zig build && zig build test green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>