Reconfiguring `import keep filtered` from off to on does not preserve already rejected routes

[水兵]

# Title

Reconfiguring `import keep filtered` from off to on does not preserve
already rejected routes

# Description

Changing a channel from `import keep filtered off` to `import keep filtered
on` does not cause existing rejected routes to be reprocessed. Two
histories with the same final static route, import filter, and `import keep
filtered on` setting end with different filtered-route state.

When the route is present before `import keep filtered` is enabled, it has
already been dropped and is not re-fed after reconfiguration. When BIRD
starts directly with the final configuration, the same route is retained as
filtered.

# Version

Reproduced with:

```text
BIRD 2.18+branch.master.f0f859c26cf9
```

# How to reproduce

1. Use a single BIRD instance. No external peers are required.

```text
+-------------+
| r1 |
| |
| BIRD static |
+-------------+
```

2. Configure a static protocol route:

```bird
protocol static static_reject {
ipv4 {
import filter {
if net = 198.51.80.0/24 then reject;
accept;
};
import keep filtered off;
};

route 198.51.80.0/24 unreachable;
}
```

3. Start BIRD and confirm the rejected route is not retained as filtered:

```bash
birdc show protocols all static_reject
```

4. Reconfigure only the channel option from `import keep filtered off`
to `import
keep filtered on`, keeping the same static route and same import filter.

5. Check the protocol counters again:

```bash
birdc show protocols all static_reject
birdc show route filtered protocol static_reject all
```

6. As a control, restart BIRD directly with the final configuration (`import
keep filtered on`) and the same static route/filter, then compare the same
outputs.

# Expected behavior

Because both histories end with the same route, the same import filter, and
`import keep filtered on`, BIRD should retain the rejected route as
filtered in both cases. The protocol route counters should report one
filtered route.

# Actual behavior

History A reports no retained filtered route:

```text
Routes: 0 imported, 0 filtered, 0 exported, 0 preferred
```

History B reports the expected filtered route:

```text
Routes: 0 imported, 1 filtered, 0 exported, 0 preferred
```

# Additional context

`import keep filtered` changes how rejected routes are represented in
channel state. The suspected path is `nest/proto.c:channel_reconfigure()`,
which handles several channel changes but does not appear to trigger a
reload/refeed for `in_keep_filtered` changes. `nest/rt-table.c:rte_update2()
` decides whether rejected routes are dropped or retained as `REF_FILTERED`
based on `c->in_keep_filtered`, so changing that flag without refeeding
existing routes leaves history-dependent filtered state.
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