MAVLink: refine reconnect STATUSTEXT (per-peer tracking, snapshot rate limit)

[xznhj8129]

Follow-up to the reconnect STATUSTEXT handling merged in #11715, addressing review feedback on the reconnect-detection design.

The original commit tracked heartbeat presence with a single timestamp per physical port and sent an arming-reason snapshot on every judged reconnect. Review raised two soundness issues with that:

• a steady peer on a port masks a newly joining peer behind the same port (their heartbeats keep the single timestamp fresh), and a peer failing over to another port is never noticed;
• MAVLink leaves heartbeat rate channel-dependent, so a peer heartbeating slower than the 5 s gap threshold is classified as reconnecting on every beat, resending the arming notice each time.

Changes

• Per-peer tracking. Heartbeat presence now lives in the route-table entry (sysid/compid), not per port. A steady peer no longer masks a new one; a peer moving to another port registers as a reconnect there. If the route table is full the new peer degrades gracefully to the pre-existing broadcast-only behaviour.
• Snapshot rate limit. A 10 s per-port floor on arming snapshots, so a slow-heartbeat peer can't elicit a resend on every beat.
• Dropped the enable-transition snapshot. It fired into a port nobody had connected to yet (boot) and duplicated the first-heartbeat snapshot on shared-port enable. Reconnect is now driven purely by the heartbeat signal; the enable transition still resets per-port one-shot state via the port reset path.

Testing

• SITL builds clean with warnings-as-errors; mavlink_unittest 38/38 at this level (no new unit tests — behaviour is timing/multi-peer and covered by the live rig below).
• Live SITL + pymavlink: existing reconnect regression 6/6, plus new cases — a 6.5 s-period peer is capped to 3 arming sends across 5 beats (was 5), and a second sysid joining a busy port gets its own snapshot while the steady peer produces none.

Built on the mav/04-streams-protocol content already in maintenance-10.x via #11715, so this shows a single commit.

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