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Johnathan Corgan
49bd210480
Replaces the unconditional `CoordCache::clear()` calls at parent-switch, become-root, and loop-detection sites with two targeted invalidation methods scoped to what actually makes an entry stale: - `invalidate_via_node(node_addr)`: drop entries whose cached destination ancestry contains `node_addr`. Used at parent-position- change sites — our prefix changed, so destinations downstream of us have stale-prefix coords. - `invalidate_other_roots(current_root)`: drop entries rooted under a different root than the current one. Used at root-change sites. Under the previous global flush, parent switches blanked the cache across the board, leaving `find_next_hop` returning `None` for every non-direct-peer destination until the cache passively re-warmed via incoming TreeAnnounces / SessionSetup. Surgical invalidation preserves entries that remain correct after the topology change. The cached coord describes a destination's tree position; that position only goes stale relative to our own routing decisions when our own prefix changes (entries we are downstream of) or the root changes (entries in a different tree). Peer removal does not invalidate cached coords: `Node::find_next_hop` recomputes the next-hop decision on every call against the current peer set, bloom filters, and tree state, and Discovery already triggers on `no route to destination` errors when a destination becomes unroutable through us. The peer-removal site retains the original "no cache invalidation" behavior. Each method returns the count of entries removed for observability. Unit tests cover each method against the cases enumerated in the acceptance criterion.