Capabilities heartbeat_interval_sec sub-second values silently truncate (§6.4)

[nficano]

Category: spec-conformance Severity: major
Location: src/Messages/Session/Capabilities.php:38-39
Spec: ARCP v1.1 §6.4

What

Spec §6.4 mentions heartbeat_interval_sec as part of welcome. Modeling as int silently truncates any sub-second configured value to 0 — a 500ms configured interval would advertise 0s and confuse clients. No validation enforces a minimum.

Evidence

public int $heartbeatIntervalSeconds = 30,

Proposed fix

Either document and validate the field as ≥1 second, or change to a float/explicit milliseconds field on the welcome payload.

Acceptance criteria

• Setting an interval less than 1s either errors or is rounded to 1s consistently with the wire field.