fix: declare plugin interval so stale data waits two full intervals

The server inferred each plugin's collection interval from the gap between
the last two received PLG samples, then expired data at interval * 3. After
an outage this guess was wrong: the request_update re-send of collect-once
InfoPlugins produced two close samples, yielding a tiny inferred interval
that purged permanent info data minutes after recovery.

Clients now declare each plugin's interval in the PLG message (_interval).
The server uses it directly (from the first post-recovery sample), expiring
at interval * 3 so live data survives at least two full intervals; interval
0 marks collect-once InfoPlugins that never go stale. Legacy clients omit
the field and fall back to the previous inferred-gap behavior.

Adds _interval to all three clients: hbc, hbc_mini.py, hbc_mini.c.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/hbc_mini.py

@@ -955,7 +955,7 @@ async def _run_info_plugins(conn: AsyncConnection, plugins: List[Plugin]):
         try:
             data = await plugin.collect()
             if data:
-                await conn.sendto({"plugin": plugin.name, **data}, "PLG")
+                await conn.sendto({"plugin": plugin.name, **data, "_interval": plugin.interval}, "PLG")
                 log.info("sent %s", plugin.name)
         except Exception as e:
             log.error("%s collect: %s", plugin.name, e)
@@ -968,7 +968,7 @@ async def _run_monitor_group(conn: AsyncConnection, plugins: List[Plugin], inter
             try:
                 data = await plugin.collect()
                 if data:
-                    await conn.sendto({"plugin": plugin.name, **data}, "PLG")
+                    await conn.sendto({"plugin": plugin.name, **data, "_interval": plugin.interval}, "PLG")
                     log.debug("sent %s", plugin.name)
             except asyncio.CancelledError:
                 raise