[jamiecounsell]

Note: this PR is stacked on #99

The link watchdog was still marked as TODO. Without it, a link request with no reply stayed pending forever, initiators never sent keepalives, and dead links were never found stale or closed.

I ported the reference loop as one pass per link, run from Transport::jobs() for each pending and active link:

• pending/handshake links time out
• active links send keepalives and go stale
• stale links close after the grace period

The watchdog depends on three values that nothing was setting, so this sets them:

• the keepalive interval, now derived from the RTT when a link comes up, as in the reference (adds KEEPALIVE_MIN/KEEPALIVE_MAX; KEEPALIVE keeps its value)
• the time the last keepalive was sent
• the time of the last proof received on the link

The watchdogs run after the job loop, not inside it. jobs() sets _jobs_running while its loop runs, and outbound() sleeps until it's cleared. The watchdog sends packets (keepalives, link closes), so calling it inside the loop would deadlock: outbound() waits for the loop to finish, and the loop waits for outbound(). The existing resource watchdogs had the same problem when re-requesting a missing part, so I moved them after the loop too.

Testing: running in SolarOS on a ThinkNode M9 (ESP32-S3, ESP-IDF) against Python RNS nodes over LoRa, exchanging LXMF messages. Native tests pass, and the series builds for ttgo-lora32-v21 and wiscore_rak4631.

Fixes #93

[attermann]

Thanks for this, and for the detailed write-up and the on-hardware testing against Python RNS. Porting the link watchdog fills a real gap, and running it after the job loop to avoid the outbound() deadlock makes sense.

Before this can go in, a few things need sorting out:

1. The contribution policy asks that behavior differing from the Python reference be guarded and documented (section 5.4). Two places differ:

   • The STALE→CLOSED transition now sends a LINKCLOSE packet, which the reference doesn't do at that point.
   • The ACTIVE check also fires on last_outbound + keepalive.

   Please either match the reference (including links to relevant reference Python code) or put these behind an RNS_* guard that's off by default.

2. PacketReceipt::validate_link_proof now validates proofs for real (it was if (false)) and records last_proof. That looks like the right fix, but it's a separate behavior change from the watchdog. Please call it out explicitly in the description, or split it into its own PR.

3. The PR is stacked on Validate link proofs #99, so please rebase onto master once that lands. The branch also predates the latest release, so a rebase would be good to confirm nothing has shifted underneath it.

4. A native test for the pending-timeout and stale→close paths would be very welcome.