Detect changes to interfaces in the dynamic store

[dlon]

This PR makes a few changes:

• Use the primary interface as the "best (scoped) default route", or else the first active interface in the network service order. (Same as before, but the flow is much easier to understand.)
• Check whether the "best default route" has changed in response to any changes to network services in the dynamic store.
• Check whether the "best default route" has changed when the user tries to connect in the offline state.
• Log all the things.

Fix DES-398
Fix DES-396

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This change is 

[linear]

DES-396 SCDynamicStoreBuilder panics during shutdown

DES-398 App gets stuck in offline state

My best guess is that we must react to dynamic store key changes.

[pinkisemils]

Reviewed 2 of 7 files at r1, 4 of 6 files at r2, 3 of 3 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

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talpid-core/src/offline/macos.rs line 112 at r3 (raw file):

                route_event = route_listener.next() => {
                    let Some(event) = route_event else {
                        break;

Should it break or continue in this case? Feels like this could be coalesced with let Some(state) = weak_state.ugprade()... here.

[dlon]

Reviewable status: complete! all files reviewed, all discussions resolved

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talpid-core/src/offline/macos.rs line 112 at r3 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

Should it break or continue in this case? Feels like this could be coalesced with let Some(state) = weak_state.ugprade()... here.

Definitely break -- this should only return none when the route manager is down.