Constant H2_MIN_BUFFER_SIZE 

pub const H2_MIN_BUFFER_SIZE: u64 = 16_393;

minimum buffer size required when any HTTPS listener advertises H2 ALPN.

RFC 9113 §6.5.2 caps SETTINGS_MAX_FRAME_SIZE at 16 384 bytes by default; the on-wire H2 frame header is a fixed 9 bytes (§4.1), so the kawa storage must be able to hold 16 384 + 9 = 16 393 bytes before forwarding. A smaller buffer_size causes the H2 mux to deadlock on full-size frames (no panic, no obvious log) until the session timeout fires. Validated at config-load time in ConfigBuilder::into_config so a typo in TOML is rejected at boot, not discovered under traffic.